A modular ankle-foot orthosis includes a lower leg and ankle support and a plantar foot support. The two components may be secured together to provide more complete support for a patient, as required. The two supports may be attached in various ways, such as a bottom heel clip extending from the foot support component, which secures through a passage in the heel of the lower leg and ankle support component. A tongue extends from the foot support clip, to capture the material at the edge of the lower leg and ankle support passage between the tongue and the foot support plate. The two components are quickly and easily assembled together and disassembled as required, to provide more versatile treatment for the patient. The complete structure, or any components thereof, is sufficiently thin to allow wear within a conventional walking shoe or the like.