The field of developmental biology has a history that spans the last 500 years. Within the last 10 years, our understanding of developmental mechanisms has grown exponentially by employing modern techniques of genetics and molecular biology, frequently combined with experimental embryology and the use of molecular markers, rather than solely morphology, to identify critical populations of cells and their state of differentiation. Three main principles have emerged. First, mechanisms of development are highly conserved, both among developing rudiments of a variety of organ systems and among diverse organisms. This conservation occurs both at the level of tissue and cellular mechanisms, and at the molecular level. Second, the development of organ rudiments is influenced by surrounding tissues through interactions called inductive interactions. Such interactions are mediated by highly conserved growth factors and signaling systems. Third, development is a life-long process and can be reawakened in events such as wound healing and regeneration, and in certain diseases. Advances in understanding normal development provide hope that diseases in which development runs amuck, such as cancer, may soon be preventable and fully treatable. Supported by NS 18112 and DC 04185 from the NIH.