The development of the mammalian preimplantation embryo in vitro occurs more slowly and less successfully compared to development in the uterus. The fact that it can occur at all in a defined protein-free medium suggests that the process is autonomous. Accumulated evidence indicates that a number of peptide growth factors contribute in an autocrine fashion to preimplantation development. Other growth factors are maternally derived and act in a paracrine manner on the embryo. Some of these factors such as insulin-related factors stimulate growth preferentially, but others such as epidermal growth factor (EGF) play more important roles in differentiation. Several cytokines appear to be implicated in peri-implantation events and in maternal-fetal interactions. At this stage, the data are mostly descriptive. Are all these different growth factors and receptors necessary for early development? Some implications of apparent redundancy of gene expression are discussed and future studies are predicted.