The carboxylic acids include alpha-hydroxyacids (AHAs), polyhydroxy acids (PHAs), aldobionic acids (ABAs), retinoic acid, vitamin C and azelaic acid. They all have therapeutic actions. AHAs, PHAs and ABAs are organic hydroxyacids, a group of natural and physiological substances which can modulate skin keratinization and increase biosynthesis of dermal components. Because of these effects, AHAs, PHAs and ABAs are therapeutically effective or beneficial for topical treatment of dry skin, rough skin, acne, rosacea, warts, eczema, psoriasis and skin changes associated with ageing, including wrinkles and photoageing. In addition, PHAs and ABAs, which are antioxidants, are topically beneficial for sensitive or diseased skin and for the prevention of oxidative damage caused by UV radiation. The vitamin A derivatives, known as retinoids, include three that are found physiologically. Retinoic acid is the most potent of these in promoting proliferation and differentiation of epithelial cells, and in stimulating biosynthesis of collagen I and III. Because of these actions, retinoic acid is therapeutically effective for topical treatment of acne, actinic keratoses and photoaged skin. Vitamin C, which is l-ascorbic acid and a lactone form of 3-keto-polyhydroxy acid, is a water-soluble antioxidant. Because of this property vitamin C has been promoted for topical prevention of skin damage caused by UV radiation. Azelaic acid has been shown to normalize keratinization in the follicular infundibulum, exert an antibacterial effect against Propionibacterium acnes and inhibit melanogenesis and so has been used for topical treatment of acne and melasma. The carboxylic acids display similarities and differences in their topical actions and therapeutic applications.