The retinoids comprise a large group of natural and synthetic compounds related to vitamin A (retinol). The family name is derived from the early observation of the necessity of vitamin A for normal vision, and the association of vitamin A deficiency with night blindness (1). With the exception of the visual cycle in the rod photoreceptor cells of the retina, in which protein-bound 11-cis-retinal is reversibly isomerized to free all-trans-retinal, retinoic acid is the active retinoid for biological processes. However, the visual cycle illustrates two characteristics of retinoids that are relevant to the use of this family of molecules as tools in experimental embryology and for the interpretation of results. (1) retinoids are light-sensitive, and will undergo isomerisation or degradation if exposed to light; (2) in nature, their activity and their metabolism are associated with binding to specific proteins.