The vitamin A and the retinoids, the vitamin A derivatives play an important role in the embryogenesis, in the modulation of the growth, in the differentiation of normal, premalignant and malignant epithelial and mesenchymal cells and in the maintenance of immune response. The effects of vitamin A are executed by two different mechanisms. While the carotenoids of provitamin A activity and the vitamin A--the retinol--itself are mainly antioxidant molecules and so some of their effects are partly similar to the effects of tocopherol, the retinoids, the derivatives of vitamin A, exert their diverse effects by regulating the expression of specific genes by the aid of specific nuclear receptors belonging to the family of steroid, thyroid hormone, and vitamin D3 receptors. The retinoids play a role in the modulation of the effect of these hormones as well.