To be aware of the environment, one must sense or perceive that environment. The body senses the environment by the interaction of specialized sensory organs with one aspect or another of the environment. The central nervous system utilizes these sensations to coordinate and organize muscular movements, shift from uncomfortable positions, and adjust properly. One relevant question is “what is the relative contribution of gravity to these sensory and motor functions?” This chapter reviews the effects of microgravity on the functioning of the sensory organs primarily used for balance and spatial orientation. Disorientation and malaise so frequently encountered during early exposure to microgravity and upon return to Earth are described. Theories and actual data regarding the role of the central nervous system in the adaptation of sensory-motor functions, including the control of posture, eye movements, and self-orientation, to changing environmental gravity levels are explored (Fig. 3.1). For a comprehensive review of space research conducted in this area since the beginning of spaceflight, the reader is referred to the book on Neuroscience in Space (Clément and Reschke 2008).