The Manaslu Trek (or Manaslu Circuit Trek) is 14-day trek which circumnavigates the 8th highest peak in the world (8,156 m). The trek has all the elements of other treks in Nepal including both Hindu and Buddhist culture, rhododendrons and wild flowers, raging rivers, precarious bridges and stunning mountain scenery.
Just over 2,000 people complete the trek per year with more than half visiting in October. This is substantially fewer than visiting Annapurna, Everest or Langtang. There are two main reason for the fewer numbers.
The first reason is that the trek used to require camping, as there were too few tea-houses along the trail. In particular there was no lodge below the Larkya La (a pass at 5135m) requiring an enforced night of camping. This is no longer the case. Since 2010 there is a tea-house below the pass and all the way up from the starting point. A few of the tea-houses are still basic compared to counterparts on the Annapurna Circuit, but most are comfortable.
The second reason is that the trek has restricted area status. This means it requires a special permit (see below), that the group have two or more members and that a registered trekking guide accompanies the group. In 2011, for Nepal Tourism Year, the permit fee was reduced to US$50 per week.
These two factors together have made the trek affordable for budget travellers and it is now being referred to as The New Annapurna Circuit.
You have to arrange this trek though a registered trekking agency, even though you may just be two people with a single guide. The agency is necessary for arranging the permit, as individual tourists are not permitted to do that.
In addition to the restricted area permits are permits for two national parks and a general trekking permit:
Prepare as you would for any other trek equipment-wise. Water purification such as iodine or a portable filter will be needed. Get fit.
The trek generally starts at either Arughat. This is a simple 5-6 hour micro-bus ride from Kathmandu costing around US$7.
Check the online Manaslu Trek map  to understand the locations mentioned in the itinerary.
Travel from Kathmandu to Arughat [600m good lodges]. The bus may take you further than Arughat depending on the state of the road ahead. Sleep there or continue to Arket [750m basic lodges] or Soti Khola [800m basic lodges] to get a head start. Otherwise the standard itinerary with days of 6 hours or less of walking is as follows:
It is also possible to start from Gorkha Bazaar, but this requires camping. A side trip to Tsum Valley is also possible, but this also requires camping or pre-arranged homestays. Itineraries can be as short as 10 days, excluding rest days, or as much as 18 days, taken at a relaxing pace with plenty of exploring.
It is also entirely possible to combine with the Annapurna Ciruit: from Dharapani, continue to Manang and over the Thorung La at speed having already acclimatized. Then fly or bus out from Jomson to Pokhara.
Popular day trips include a visit to Manaslu basecamp from Samagaon and a hike to the border with Tibet which is a 5 hour return trip from Samdo.
The principle danger is altitude, specifically going too high too quickly. Follow these instructions for altitude sickness prevention
Finally take a bus from Besisahar to return to Kathmandu, or continue onward to Pokhara.