This paper reviews 18 reports which have investigated the absolute threshold of perception of periodic linear motion. The roles of the otolith, somatosensory, and visual detection mechanisms in determining threshold are discussed. Most threshold data for oscillation at frequencies below 1 Hz reflect otolith and somatosensory detection, and show a falling threshold as the frequency rises. This is in accord with neurophysiological data of otolith and somatosensory function. The data for frequencies above 1 Hz reflect an unknown mix of visual, otolith, and somatosensory influences. These data are too heterogenous to indicate whether threshold rises or falls as the frequency of stimulation increases.