Providing patients with a leg prosthesis or a (knee) ankle foot orthosis requires an estimated guess about the effects of the prosthesis/orthosis on gait. Mostly we observe and interpret gait. For a proper gait observation, knowledge of normal gait and gait biomechanics is essential. Also, by just observing gait, we can get no information about the forces and joint moments and, therefore, a proper alignment is very difficult to perform. This contribution will focus on some basics of gait, some basics of gait biomechanics, possibilities with instrumented Gait Analysis and some examples of interventions with their biomechanical effects.