One should be aware that no one else, no hotel, no street broker, no nice person you just met, not even a trekking guide is legally authorized to organize a trek. During the main seasons the agencies run regular group treks, both tea-house and camping styles.
Only TAAN registered trekking agencies in Kathmandu and Pokhara can legally organize treks and provide the services of a guide and/or porter with insurance.
A trekking permit issued by the Department of Immigration is required to trek in any part of Nepal, except the most popular areas of Annapurna, Khumbu and Langtang/Helambu. Those areas were declared permit-free in 1999. The joy was short-lived, though, as a new system called TIMS (Trekker Information Management System) was recently created for those three areas. Be sure you have a TIMS card with you when trekking independently or organized. Individual TIMS is obtainable only from Nepal Tourism Board offices and the Trekking Agents Association of Nepal office. Not even Trekking Agents are legally authorized to obtain individual TIMS (even do many small Trekking Agents will offer the individual TIMS). Police check points and Park officers can at any time check your permits.
Several National Parks and Conservation areas like ACAP (Annapurna Conservation Area Project) and Sagarmatha National Park (Everest area) require trekkers to pay an entrance fee.
Restricted areas require the old trekking permit (but not the TIMS card), which are obtainable only through the organizing trekking agent for areas like Dolpo, Mustang, Manaslu, Kanchenjunga and other similar areas.