Static and dynamic equilibrium (posture and gait) is indispensable for a normal behavior in human. Its mechanisms are nearly the same in superior species and in man, but humans are the only ones to have acquired exclusively biped upright position and gait. Equilibrium is a sensory function involving 3 sub systems: visual, vestibular and sensori-motor (proprioceptive), controlled by cerebellum and cerebral structures. Information coming from those 3 sub systems must be concordant. In case of discordance, the patient is in a state of sensory conflict. This conflict can induce disequilibrium or true vertigo. After a recall on equilibrium, the authors report the results of an epidemiologic study upon 5298 patients suffering of balance disorders and treated by an alpha-blocker: nicergoline (30 mg/day during 6 months).