Thamel is the hub of Nepal's trekking industry, with hundreds of agencies competing for business. The quality varies enormously. For standard routes like the Everest Base Camp or Annapurna Circuit, a reputable agency should be able to provide a licensed guide, all necessary permits (the TIMS card and national park entry permit), equipment rental, and accommodation bookings. For technical climbs or off-trail routes, experience and licensing become critical.
Stick to agencies that are members of the Trekking Agents Association of Nepal (TAAN) and ask to see your guide's licence. The Kathmandu Environmental Education Project (KEEP) runs a trekker's information centre in Thamel with unbiased advice on responsible trekking and can recommend vetted agencies. Many independent travellers also choose to hire a licensed porter-guide directly at the trailhead rather than through a Thamel agency.