The intention of this paper is to give an account of the physiology of complex sensory structures in the skin, using information gained from the study of simpler situations. The principal methods used to establish the functional characteristics of identified cutaneous mechanoreceptors have been electrophysiological and morphological. In particular, it has been important in this work to be able to record electrically from single afferent units and then to study the morphology of the receptor giving rise to the afferent discharge (Iggo, 1966). For example, Loewenstein (1966), following the earlier work of Gray and Sato (1953), excised Pacinian corpuscles and was then able to analyse the functional characteristics and the relation between morphology and physiology of these receptors in much more precise detail. In such studies the receptor was removed from the animal and as a result rigorous control of the physical conditions was possible. Another identified afferent unit in the skin is the Merkel touch spot which has been investigated electrophysiologically in situ in anaesthetized animals and which has the advantage that in some species (e. g. cat) the receptor is visible on the surface of the skin. The ultrastructure of the receptor can then be established after fixation of the skin in situ (Iggo and Muir, 1969). With receptors not visible on the surface of the skin other techniques have been successfully developed in order to locate the receptor for morphological studies. A recent example of such a method is the insertion of fine wires or hairs into the skin, to mark the receptive field. These markers or the holes they make are then visible in the fixed tissue (Chambers et al.,1972; Hensel, 1973; Gottschaldt, 1974).