Transdermal drug delivery offers an attractive alternative to the conventional drug delivery methods of oral administration and injection. However, the stratum corneum acts as a barrier that limits the penetration of substances through the skin. Application of ultrasound to the skin increases its permeability (sonophoresis) and enables the delivery of various substances into and through the skin. Ultrasound has been used extensively for medical diagnostics and to a certain extent in medical therapy (physiotherapy, ultrasonic surgery, and hyperthermia). The generation of ultrasound and mechanism of sonophoresis with particular emphasis on the role of cavitation (both inside and outside the skin), thermal effects, convective transport, and mechanical effects also included. There are certain findings in the field of sonophoresis, namely transdermal drug delivery and transdermal monitoring. The article also encompasses a discussion on the variation of sonophoretic enhancement from drug to drug, possible applications of sonophoresis in near future, some commercially available sonophoretic systems and future trends. Particular attention is paid to proposed enhancement mechanisms and future trends in the field of cutaneous vaccination and gene delivery. Key words: Ultrasound, Sonophoresis, Transdermal, Stratum corneum, cavitation, thermal effects, convective transport, and mechanical effects, Hyperthermia.