Retinoids comprise both naturally occurring and synthetic molecules that have a structural resemblance to all-trans-retinol, which by definition is vitamin A. Thus, the term retinoid is used to refer both to vitamin A and its metabolites and to chemically-related compounds that have been specifically synthesized as potential pharmacological agents for treating disease. It has long been known that the naturally occurring retinoids are required for maintaining immunity, barrier function, male and female reproduction, embryonic development, cognitive function, and vision. Over the last decade it has become increasingly clear that aberrant actions of naturally occurring retinoids and retinoid-related proteins are associated with development of metabolic disease, including obesity, type II diabetes, liver disease and cardiovascular disease. At the cellular level, retinoids are needed for maintaining normal cell proliferation, differentiation and apoptosis. Consequently, for over 50 years there has been much research interest focused on the use of both natural and synthetic retinoids in the treatment of proliferative disorders, especially cancers and skin disease.