Efferent sensory systems have emerged as major components of processing by the central nervous system. Whereas the afferent sensory systems bring environmental information into the brain, efferent systems function to monitor, sharpen, and attend selectively to certain stimuli while ignoring others. This ability of the brain to implement these functions enables the organism to make fine discriminations and to respond appropriately to environmental conditions so that survival is enhanced. Our focus will be on auditory and vestibular efferents, topics linked together by the inner ear connection. The biological utility of the efferent system is striking. How it functions is less well understood, and with each new discovery, more questions arise. The book that is proposed here reflects our vision to share what is known on the topic by authors who actually have made the observations.