When skin is mechanically stimulated, differentreceptor systems will be activated depending upon the type of skin stimulated and stimulus characteristics such as rate and amplitude of displacement and repetition frequency. The general properties of the primary mechano-receptive units are (for the rapidly adapting intracutaneous receptors) low thresholds, restricted receptive field, relatively good rate sensitivity (in terms of repetitive impulse discharge), and poor capacity of tracking on high-frequency repetitive stimulation; (for the Pacinian corpuscles) high critical slope, very low displacement threshold, diffuse receptive field, poor rate sensitivity, and good frequency-following capacity; (for the tactile spots in hairy skin) low displacement threshold, small receptive field, good rate sensitivity, and good frequency-following capacity. When tactile displays are designed, the stimulus parameters will have to be chosen according to the psychophysical functions as well as to the receptor properties, in order to obtain a device that is optimal from a discriminatory point of view.