Retinoids are a group of naturally occurring and synthetic compounds with vitamin A-like biological activity. They play an important role in vision, reproduction, growth, and epithelial cell differentiation. Recent discoveries of specific retinoid cellular binding proteins and nuclear receptors have led to a better (although not complete) understanding of the complex mechanisms of retinoid action. Numerous clinical studies have demonstrated beneficial effects of retinoids on skin diseases such as acne, psoriasis, ichthyoses, keratodermas, skin cancers and their precursors, as well as a reversal of the effects of photoaging.