Retinoids are natural and synthetic analogues of vitamin A. They have a marked influence on the differentiation of epithelial tissues by modifying membrane glycoconjugates and by acting through cytosolic and nuclear mechanisms like steroid hormones. Their clinical and biological toxic effects are numerous, but they are usually benign and reversible. However the possibility of teratogenic effects requires close monitoring of female patients. Etretinate is particularly effective in disorders of keratinisation (psoriasis, ichthyosis) whereas isotretinoin is the best known therapy of severe acne. Moreover the possible role of retinoids in the prevention and treatment of certain cancers gives hopeful prospects.