Etienne-Jules Marey introduced gait motion analysis at the end of the XIXth century. It was rapidly adopted by clinicians and Charcot used it at the beginning of the XXth century in La Salpêtrière. Motion analysis was widely used after the first optoelectronic system was built by Furné in 1968. The optoelectronic system calculates the orientation of each body segment in the space after the determination of the space co-ordinates of cutaneous markers placed over them. It is particularly useful for spastic gait. Many disturbances of kinematics and kinetics have been described during spastic gait. They are the consequences of spasticity and other motor and sensory deficits of central nervous system lesions. Motion analysis must be coupled with electromyographic recording of spastic muscles activity which, with kinetic analysis, enables distinguishing the mechanisms underlying gait disturbances. Motion analysis provides clinicians with an indispensable tool for understanding spasticity and evaluating therapeutic efficacy.