Development of mammalian conceptuses (embryo and associated membranes) in utero is an extremely complex, programmed orchestration of cellular activities that include proliferation, differentiation, migration, and death. Failure of conceptuses to develop is a significant, though not well-understood, problem in human and animal reproduction. Increased understanding of molecular and cellular processes in these events is ultimately required to determine the mechanisms responsible for developmental failures. Peptide growth factors, their associated proteins (carrier proteins and membrane receptors), and proto-oncogenes (the normal, cellular counterparts of retroviral oncogenic proteins) are intimately involved in normal and abnormal cellular growth regulation. This paper highlights recent work which suggests the involvement of these proteins in critical aspects of conceptus growth and uterine-conceptus interactions during the peri-implantation period.