The function of the vestibular system is not as obvious as those of vision, hearing, touch or smell. Vestibular dysfunction, however, is clearly apparent where lesions are present. It is probably for this reason that the vestibular sense was not discovered until the nineteenth century and that clinicians have continued to playa major role in basic vestibular research right up to the present. The relationship between basic and clinical research is certainly stronger in the vesti­ bular field than in that of tactile sensation, for instance, as testified by the work of clinicians as MENIERE, BREUER, BARANY, DEKLEIJN and FRENZEL. In this respect the situation is similar in vestibular physiology and in endocrinology, and for the same reason. This second part of the vestibular volume of the Handbook of Sensory Physio­ logy will be of interest to neurologists, otologists, neurosurgeons, ophthalmologists and physiotherapists on the one hand, and psychologists, physiologists, engineers and aviation specialists on the other. For a full understanding of Part 2, it is necessary to have assimilated the basic anatomy, physiology, and biochemistry of Part 1.