Europe has a number of low cost airlines, the largest and most established being easyJet , RyanAir , germanwings  and Air Berlin . These airlines have stirred up air travel within Europe by dramatically cutting fares.
The European Open-Skies Treaty of 1992 blew the lid off the system in place before, where national government would restrict access to their airspace to expensive 'flag-carriers', such as British Airways  or Lufthansa . This enabled airlines to fly anywhere they wished in the European Union without government approval.
Ryanair was the first airline in Europe to try this model, and now has many followers offering low fares across the continent. These are boom times for cheap air travel in the European Union, with fares on some routes as low as €10 (£7, US$12) one-way including tax (though average fares for international flights are around €80 one-way).
Do not overlook other European airlines: for example depending on the your destination, dates and time budget the normally expensive Swiss airline can be cheaper than a so-called discount airline, when all costs and times are included from source to destination. Another example is OLT, which offers business flights, can be cheaper than any alternative airline for some legs.
Most discount airlines in Europe sell their tickets exclusively over their website or over the phone, and tickets are not available via travel agents. Most are ticketless; you simply turn up at the check-in desk or even just at the departure gate with your passport and confirmation number (and print-out of your e-ticket). A credit or debit card is a very good idea for booking tickets. Most discount airlines sell their tickets as single journeys only.
The pricing structure is complex, with fares fluctuating strongly according to demand, often on an hourly basis, and the same rule "get as much money as a traveler is ready to pay" that was invented by traditional carriers applies. There are no hard and fast rules for obtaining the cheapest fares. In fact, fares can vary from as little as £1 or £2 on special promotions, right up to £500 - such as a London-Geneva return flight, during the February half-term weekend (winter holidays in most of the schools).
The following will however increase your probability of obtaining very inexpensive fares:
Do fly mid-week
Do fly early in the morning or late at night
Do fly in low season (Spring and Autumn)
Do make use of sales. These sometimes appear 3-5 weeks prior to departure, however this is by no means guaranteed.
Don't fly during public holidays.
Don't book your ticket less than two weeks in advance
Opt for return tickets, but keep in mind, in most cases airlines will charge extra fees for changes of date or time.
For most traditional airlines it is possible to book a flight from A to B with a connection at C on a single ticket. For many low-cost carriers, this is not possible as they only offer single "point to point" flights. To make a connection with a low-cost carrier, you need to purchase two separate tickets, one from A to C, and another from C to B, and these count as separate contracts. Connecting low-cost flights can save on cost but it has a few disadvantages:
You are not guaranteed to make the connection to your final destination. If your first flight is delayed, so that you miss the connection, it is your responsibility. The airline fulfilled the first contract by bringing you to the connection point albeit delayed, and it is your problem that you failed to arrive at the airport in time to get your second flight. Travel insurance may sometimes cover an event like this, paying for another ticket on a later flight, but only if you have put up a safe connection time.
All checked luggage will need to be picked up at your connection point as if that were your final destination. It then needs to be checked in again as if you are departing from that airport.
However, some low-cost carriers want a share of the market with transporting connecting passengers, and have policies which allow for connecting tickets. Note that connecting flights here refers to connections to another flight on the same airline.
Planning low-cost flight connections can be complicated and requires access to vast amounts of data.
Very few low-cost carriers offer connecting tickets to different airlines. (Many traditional carriers don't offer such tickets either although interline agreements often exist.)
Ryanair does not offer connecting tickets, and discourages people from flying with them if they need to connect.
EasyJet do not offer connecting tickets, and advise passengers who need to connect to calculate a two hour connection time.
Air Berlin offers connecting flights on their website and codeshares with some members of the oneworld Alliance
Norwegian generally operate point-to-point and recommend a connection time of 2 hours. If you have calculated two hours but still miss the connection, Norwegian will rebook you to a later flight subject to available space. Connecting flights can sometimes be booked on a single ticket online ("onlining") for a surcharge of 40 NOK, and in those cases they are responsible for bringing you to your destination.
Wizzair do not offer connecting flights, and accept no liability for missed connections. Passengers are advised to calculate "sufficient time".
German Wings offer connecting flights, and can often check your luggage through to your destination.
bmibaby do not offer connecting flights, and do not accept responsibility for missed connections.
Flybe offer connecting tickets, and will try to reaccommodate passengers onto the nearest available flight if a connection is missed.
Jet2 do not offer connecting flights, and accept no responsibility for missed connections.
Monarch Airlines do not offer connecting flights, and accept no responsibility for missed connections.
Smart Wings do not offer connecting flights unless expressly stated otherwise. They recommed a two hour connecting time, but do not guarantee the connection.
Transavia do not offer connecting flights on their website. Their general conditions of carriage say "If a passenger is prevented from traveling within the period of validity of the ticket because Carrier: ... (5) causes the passenger to miss a connection; ... the validity of such passenger’s ticket will be extended until Carrier’s first flight on which space is available for that passenger in the class of service for which the fare has been paid."
Meridiana do not offer connecting flights on the website; the conditions of carriage do not cover the issue.
Iceland Express do not offer connecting flights per se, though they have a service which sets up connecting routes. The contract of carriage accepts no responsibility for missed connections.
Fly Low-Cost Airlines You can search flights from most of low cost airlines.
The Low-Cost Airlines Guide Has useful pages on low cost carriers sorted by country. 
Viagens low cost You can search flights from every European low cost airlines.
Discount airlines are often much more strict about their fares. For example, you have to pay an extra fee for your baggage, e.g. 15-20 € one-way at Ryanair or 8 € at Germanwings. While in traditional airlines they usually allow some baggage over the weight limit, WizzAir will charge you €6 for each kg over the limit. Also, some airlines have lower limits than the usual 20 kg. A few kilograms of weight can double your ticket price. Check your terms carefully and weigh your luggage before a journey. You should weigh and measure your hand luggage too; some discount airlines have lattice boxes at the gate to measure your hand luggage.
Food is usually not served during the flight, or it is available for a fee. It's best to bring your own food and water. Liquids are allowed through security only in bottles of 100 mL or less (and contained in a clear, 1-liter bag), but you can bring an empty bottle and fill it up at a drinking fountain or restroom tap. Alternatively, buy bottled water after security check.
In-flight entertainment isn't normally provided either. Again, bring your own (laptop, music player, book or magazine)--although electronic devices are not allowed during landing and takeoff, as they are not with traditional carriers.
Most discount airlines try to lower airport fees, so they often use smaller and more distant airports, sometimes quite far away from the city they state they fly to. For example Paris Beauvais Airport is some 90 km from Paris, bus costs about €14 one-way and it takes about 1h15 to get to Paris (taxi would be €130-150 one-way), "Frankfurt-Hahn" is near the hamlet "Hahn", is nearer to the Netherlands (about 100 km) than to Frankfurt (about 125km). Especially Ryanair (the biggest airline) uses nearly exclusively such airports.
Discount airlines do not wait for late running passengers, since an idle plane waiting for a passenger costs money. Check in desks shut promptly at the advertised time. If you are one minute late, they will not let you check in. Also, if you do not get to the boarding gate in time, you may find the plane gone and your luggage sitting on the ground. In these circumstances you will not get a refund, but you may get a transfer to a later flight if there is room.
Many airlines have changed their schedule with as little as week before departure, so the flight is up to 10 hours earlier/later than in the original reservation. Options they typically give are: accept the change; re-book on a different flight (normally you still have to pay the difference in ticket price but no fee); or accept a refund. Note that purchasing another ticket with either that airline or another at a week's notice may be very expensive relative to your original purchase. When flying low cost it is always better to have good cancellation policy from the supplier connecting with the flight (next flight in your itinerary; hotel at your destination; car rental at the destination airport etc).
Many discount airliners are "point-to-point" airlines, and do not sell connecting tickets if you need to take two planes to reach your destination. This means you might need to collect your luggage and check it in again for the next leg of the journey, and they do not take responsibility if you miss your connection, even if your connecting flight is with the same airline. This could force you to purchase a new ticket for the next flight. Some low cost carriers (notably Air Berlin) do offer end-to-end tickets, but normally only if you book the entire journey as a single ticket.
Especially Ryanair desires a internet-check-in. Checking in at the airport costs a surcharge of about 40 € one-way per person.
Other points to consider
Do check out deals from the traditional carriers as well, especially on return trips they may have offers rivalling those of the discount carriers. Following competition from discount airlines, traditional carriers such as BA have also cut their fares on competing routes, and are often only about 20% more expensive than discount airlines, a price worth paying if the journey to the airport is cheaper and faster. Sometimes they can even be cheaper than discount airlines, especially during public holidays.
Also check high speed rail connections in countries where they are available (France, Spain, Germany, Italy, UK). Railway companies have started to offer discounted advance fares (as low as €20 one-way) in response to competition from budget airlines. Travelling by high speed train can often be faster and cheaper than by discount airline, once you take into account the cost and time needed to get to the airport, as well as lenghty embarking/disembarking procedures.
Contrary to public perceptions, most budget airlines have an excellent safety record.
The flight frequency and departure/arrival times are usually worse on discount carriers compared to traditional ones.
Traditional carriers will rebook you on the next available flight for free if something happens. Low cost carriers often charge for this or force you to buy a new ticket.
easyJet plane taking off
easyJet  carried 30.3 million in 2005 making them just smaller than Ryanair and the 7th busiest airline in Europe. Fares are priced as single segment one way trips. Their website allows you to book multiple flights simultaneously however, and even allows you to exchange a flight you have purchased for a different flight of your choice on their website providing a partial refund (e.g. changing to a flight on a different date and/or with different passenger names). If you change planes at an Easyjet hub you must collect your luggage and check it in again at the hub. You can book a return at the same time as the outbound but you get no discount for doing so. Some of the advanced features on the Easyjet website are only available if you create an account for yourself on the website.
Following Ryanair, easyJet has no free weight allowance for luggage, and charge for all checked luggage. However, they do not charge extra for printing a boarding pass at the airport.
EasyJet operate an ever-expanding network, keep your eyes peeled to their site. Unlike Ryanair, easyJet tend to operate out of principal airports, such as Barcelona rather than Barcelona Girona, in Spain.
Tickets can range from €20 to €420, all inclusive one-way.
There are no advance seat assignments. However, Easyjet offers the option for pre-boarding for a small surcharge. This could be well worth the money for some travellers. However, this only works where gates lead directly to the plane: if you have to take a bus to the plane, your pre-boarding will only get you on the bus early. After that, it's a scramble with everyone else, pre-boarding or not. Requests for refunds for Speedy Boarding where planes are boarded by bus are generally met with refusal via form emails.
Ryanair , with 65 Million passengers in 2009 is Europe's largest low-cost carrier, the 3rd largest airline in Europe in terms of passenger numbers and the largest in the world in terms of international passenger numbers. Ryanair carries more international passengers than any other airline. Fares are priced as single segment one-way trips. If you wish to change planes at one of Ryanair's hubs, then you must book the two segments separately. Luggage is not transferred and must be collected and checked in again. You can book a return at the same time as the outbound but there is no discount for doing so. If you miss a second flight due to a delay in the earlier Ryanair flight, you will not get a refund for the missed flight and will be forced to buy a new ticket at the applicable price, which may be higher than you originally paid.
Tickets start from €0.01 all inclusive one-way during promotions, however always check the full final cost of the fare including all "taxes" and "fees" before booking. Most Ryanair flights that are advertised for €0.01 end up costing at least €10 after such fees, even before airport tax. When booking online (the only reasonable method and possibly the only one available at all), the final cost of your reservation may be difficult to find out until you have confirmed everything, including the payment details. Ryanair charges a credit card fee of €6 per person and segment, which can only be avoided with a prepaid Mastercard credit card. The average price one-way is about €40-50 inc. baggage and all fees and taxes.
Ryanair has a lot of add-on fees such as €15-20 per bag per segment. The fee for overweight luggage is €40 per kilo. There is no free checked baggage allowance on Ryanair, plus they have strict carry-on rules about cabin baggage, only one piece which may weigh no more than 10 kilograms. The cabin baggage rules are not always seem to be enforced though. Airports which are heavily dependent upon Ryanair flights tend to be stricter about enforcing cabin baggage rules and will often check that cabin baggage conforms to Ryanairs limit on dimensions, which is currently 55cm x 40cm x 20cm. Other airports, such as Düsseldorf Weeze, will ask passengers to put their cabin luggage on a set of scales, if they suspect that the luggage may be overweight.
Ryanair uses small airports that can be quite far from the the city they purport to serve so check your travelling time and cost estimates carefully. Coach tickets from airport to city centre may be available through Ryanair, but there may be local competition with more favourable schedules and/or fares. Check through the airport's website, or do an online search for "airport coach" plus the name of the airport.
Ryanair keeps extremely low prices by setting a standard customer behaviour (typically an airport to airport travel without on flight meal and hand luggage only) and placing additional fees for every addition you need. So although you may be able to pick up a €20 fare for a London - Milan flight, in-flight meals and snacks are charged at a premium and there are other charges for things like items of checked-in luggage. Ryanair's "no-frills" approach is aimed at travellers requiring a basic cheap transport service. Beware of getting on last when the plane is fully booked. Because everyone takes the maximum size hand baggage to avoid paying the suitcase fee the containers for hand baggage rapidly get filled up. If there is no room for yours it will be put underneath with the checked-in baggage and you will have to wait for it to arrive at the carousel.
In order to offer faster check-in to passengers with hand luggage only, Ryanair allows passengers to check-in in advance via their website. If booked in advance, there the fixed fare €15.00 per single trip per checked bag is €15 (€20 in July and August and for international flights to and from the Canary Islands). The cost if paid at the airport is €35. Luggage weight limits are 10kg for hand luggage and 15kg for checked luggage or 20kg for an increased charge of €25. It is possible to check in a second piece of luggage, but this is charged at €35 and limited to 15kg (€40 in July and August and for international flights to and from the Canary Islands).
Compared to most other budget airlines, Ryanair provides very limited compensation in the event of flight cancellations, despite the EU regulations. Typically, Ryanair will only provide a replacement seat on a later Ryanair flight (which can depart up to 3 days later than the original flight), or a full refund of the single journey price. Alternative travel arrangements and accommodation are not normally provided by Ryanair. Passengers wishing to return on the same day are normally forced to purchase a new non-advance ticket with a different airline, which can far exceed the price of the original ticket. Therefore, it is advisable to obtain insurance against flight cancellation when travelling with Ryanair.
Also bear in mind that since October 2009, Ryanair is very strict about checking in online and printing your boarding pass at home. You will be charged with a £40/€40 issuing fee if you do not have your boarding pass with you. You must have deposited any checked-in luggage no later than 40 minutes prior to the scheduled departure time. When flying with Ryanair it is advisable to get to the airport early as bag drop desks (as Ryanair terms them) can often have long queues.
airberlin is Europe's third largest discount airline and the second largest German carrier. It offers a huge network between Germany, Austria, Spain and other regions around the Mediterranean Sea like Greece, Tunisia and Egypt as well as throughout most European countries like France, Italy, Russia and Scandinavia. airberlin also offers some long-haul flights, for example to New York, Bangkok and Dubai.
Tickets start from €44,99 one-way including all taxes and fees, free beverages, snacks, sweets and newspapers aboard and up to 20 kg checked baggage. They can be booked one-way at no penalty. Connecting flights via their hubs in Düsseldorf, Berlin, Nuremberg (all in Germany) and Palma de Mallorca (Spain) or Munich are also available.
Some flights, especially from and to Austria, are carried out by their partner NIKI. airberlin is in the preparations to join the oneworld Alliance in spring 2012. Therefore other codeshare partners are American Airlines or S7 Airlines.
Other low cost airlines
There are 62 low cost airlines in Europe, and this number is rapidly changing. Here are a few of the biggest, grouped by their base country.
airBaltic  have a wide variety of cheap fares from Riga, which can be used as a transit point. E.g. it is cheaper to travel Odessa-Riga-Kiev with airBaltic than Odessa-Kiev directly with regular-fare airlines.
FlyBe  operates out of the UK to many European destinations
Jet2  operate out of UK airports Belfast, Blackpool, Edinburgh, Leeds/Bradford, Manchester and Newcastle to destinations throughout continental Europe.
Monarch  Operates scheduled low-cost flights from London Luton, London Gatwick, Birmingham and Manchester airports to various destinations in mainland Spain, the Balearics, Portugal, Gibraltar, the Canaries and Cyprus.
Thomsonfly  Operates from many UK airports to destinations across Europe and Northern Africa, as well as to Tel Aviv.
Thomas Cook  Offering flights from 21 UK airports to 75 destinations.
Condor  operates out of major airports in Germany mainly to holiday destinations and sells tickets starting at 29,00 € one-way within Europe, putting it into the discount airline bracket as well. Offers free food and beverages catered by Lufthansa parent.
germanwings  operate a large network out of German airports Berlin, Cologne-Bonn, Hamburg, and Stuttgart. The airline also offers guaranteed connection flights between some of its destinations. (from €20 all inclusive, one-way, to all destinations.)
TUIfly  former Hapag-Lloyd Express operate a large network out of Stuttgart, Cologne/Bonn, Hanover, Hamburg and Berlin. Flies as far as Greece and Israel. (from €20 all inclusive, one-way.)
Wizzair  is a Hungarian airline which operates out of Poland and Hungary (fares are from €20 all inclusive, one-way.) At the beginning they charged for all checked luggage, but in the last years they allow weights up to 15 kg free. Please check in advance as the policy on baggage can change without any notice
Danube Wings  is a Slovakian airline based out of Bratislava which flies to numerous destinations across Ireland, the United Kingdom, Italy, Croatia, Greece, Spain and France just to name a few. Some of the flights are summer only destinations. They have become infamous for cancelling flights with little notice and then not providing a refund. Book with them at your peril.
Norwegian  operates direct flights from Warsaw to Oslo-Gardermoen, Bergen, Stavanger, Stockholm-Arlanda, Athens, Dubrovnik, Split, Rome, Salzburg, Paris-Orly and Malaga; from Krakow to Oslo-Gardermoen, Oslo-Rygge, Bergen, Stavanger, Copenhagen-Kastrup and Stockholm-Arlanda; from Wroclaw, Szczecin and Gdansk to Oslo-Gardermoen.
Brussels Airlines , the successor of SN Brussels Airlines and Virgin Express, operates from Brussels to destinations in Italy, Spain, Greece, Germany, Switzerland, the UK and other countries. With b.light, fares start at €99 all inclusive, return. Brussels Airlines also has an extensive network of destinations in Africa.
Jetairfly  has very cheap flights to far off cities in Europe, America, the Caribbean and even Asia.
Air Pegasus is a turkish airline that flies from Paris, Frankfurt, Brussels into Asia, with connections in Istanbul. 
Blue1 , a Scandinavian Airlines subsidiary and a Star Alliance member, operate routes within Europe (mainly from Helsinki) and eight different cities in Finland. Not really low-cost, but often offers very competitive prices.
Norwegian  operates out of Norway, Sweden, Denmark and Poland. Has very cheap flights. Took over Sweden's FlyNordic in 2008. Main hubs: Oslo (Gardermoen and Rygge), Stockholm, Copenhagen and Warsaw. Domestic flights within Norway, Sweden and Denmark, and international flights to destinations in Europe (primarly from the four hubs, but also from some smaller cities in Norway). Norwegian operates in 25 european countries (plus Morocco, Egypt and the United Arab Emirates), and has flights to more than 70 destinations.
Cimber Sterling  was formed when Danish airline Cimber, bought some of the assets of the now bankrupt low cost airline Sterling. Operates out of Copenhagen and Billund to several destinations in southern Europe and Britain as well as Danish domestic and intra Scandinavian flights.
Based outside Europe
AirTransat  is a Canadian airline which operates from several Canadian cities (Calgary, Edmonton, Halifax, Montreal, Ottawa, Vancouver and Québec City) to the UK and Europe (fares are from $99 all inclusive, one-way.)
Asia & Australia
Air Asia  is a low cost airline based in Kuala Lumpur. It provides network services to Europe and Australia.
Low cost airline hubs
If you cannot find a direct flight with a low cost carrier, it may be necessary to change flights at a low cost airline hub. Make sure you leave plenty of time for connections, as you will not be refunded if you miss a flight. It may be sensible to stay overnight in a city near the hub to be sure you won't miss the flight.
List of low cost airline hubs
Airport name (airport code)
Low cost airlines
Monarch, Easyjet, Air Berlin, FlyBe, Ryanair, Thomas Cook, Jet2, Thomson Airways, transavia , Sterling