By air: Telemark is served by two airports with scheduled flights, Skien Geiteryggen Airport and Notodden Airport.
Nearby Sandefjord Airport Torp  has direct flights from many European cities. From there, take a bus to Porsgrunn, Skien or Bø. You can also take the gratis shuttle bus to the nearby train stop ("Torp Sandefjord Lufthavn")  and then the train (to Porsgrunn or Skien), but this option is mostly used by people taking the train elsewhere.
By car: E18, the main road of Eastern and Southern Norway, passes through Telemark in the districts of Porsgrunn, Bamble and Kragerø. There are several tollbooth stations, but they are of course not that expensive if you can afford renting a car and paying for fuel. E18 between Gardermoen is one of the best roads by Norwegian standards. Local roads and even other E18 stretches can require both patience and experience. During the worst summer traffic, you will need 5+ hours for a 200km trip on E18.
Lots of buses from Oslo, around 270NOK/person.
From Gardermoen Airport via Oslo, STADLER Flirt train sets (new 2012) go along the coast to Porsgrunn (about 2h35m) and Skien (2h45m). Tickets are 300+NOK, 199NOK ("Minipris") if ordering a few days ahead. Another, newish train set (from early '00s) goes to Nordagutu, Bø (2h7m from Oslo), Lunde and Drangedal. Do not put wet luggage on the overhead, as the water will drip down on you.
Ibsen's Venstøp, Skien - The place where the world-famous writer Henrik Ibsen was born.
Telemarkskanalen (The Telemark Canal). Telemarkskanalen was hewn out of the rocks by hand, and stretches 105 km from Skien to Dalen in west and to Notodden in east. The Canal consists og eight locks with a total of 18 lock chambers, and a difference in height of 72 metres. The first part was completed in 1861 (Norsjø -Skien) and the Bandak Canal in 1892. edit