This article presents a review of the literature relating to iontophoresis. This technique has been used in physical therapy to introduce ionic medications through the skin, primarily for a local effect. Recently, there has been increased interest in using this technique for the transdermal delivery of medications, both ionic and nonionic. This article includes an overview of the history of iontophoresis and a discussion of the physico-chemical and biological factors affecting iontophoretic drug transfer for both local and systemic effects. Factors affecting skin injury and techniques for optimizing iontophoretic drug delivery through the use of current modulation, electrode construction, and skin permeation enhancers are also discussed. Clinical applications of iontophoresis in physical therapy and the pharmacology of selected medications are presented. Thoughts for future potential uses of this technique and needs for further research are also discussed.