Retinoids (retinoic acid and its biofunctional analogs) are widely involved in the control of cell proliferation, cell differentiation, and embryogenic development. A series of novel synthetic retinoids (called retinobenzoic acids), which include retinoid antagonists, have been developed and have been shown to be useful tools to investigate retinoidal action molecular mechanisms. Retinoids elicit their biological effects by binding to specific nuclear receptors (RARs) belonging to a steroid/thyroid nuclear receptor superfamily. RARs act as retinoid-dependent transcription factors which bind to a specific gene site and control the gene's expression. The diversity of retinoidal actions can possibly be interpreted by considering the following characteristics, all of which are quite diversified: the structure and spatial/temporal distribution of RARs, the base sequences which interact with RARs, the cell type specifically determined hierarchy of gene expression, and the nuclear coregulators which interact with RARs. Abnormality of an RAR gene which might cause acute promyelocytic leukemia is also discussed.