The enhancement of transdermal transport by ultrasound is reviewed. After a brief discussion of the physics of ultrasound and its medical applications, the effects of high- and low-frequency ultrasound on the transport of substances across the skin are examined. The impact of low-frequency sonophoresis appears to be much more important, with significant increases in transport into and from the skin following its application. Although the mechanism of action remains incompletely defined, cavitation and thermal processes are strongly implicated.