Implantation marks a transition stage in the progress of pregnancy during which the blastocyst assumes a fixed position and begins an altered physiological relationship with the uterus, but it is not a single event, nor can it be established as occurring at a single instant in time. It is nevertheless possible to restrict our view of implantation and to discern some general features by considering that, despite variations in levels and ratios of estrogens and progesterone, structure of the endometrium, and size of the blastocyst, implantation in mammals always involves a direct interaction of the trophoblast of the blastocyst with the luminal epithelium of the uterus.

Because of recent progress in the study of cellular interactions during differentiation and interesting suggestions concerning physiological mechanisms involved in the initiation of implantation, it seems particularly appropriate to take an overall view of the morphological context of the events in the process of implantation, and to suggest in which areas these events have implications concerning the cellular mechanisms that may be involved in this crucial event in development. In addition to emphasizing general biological features, consideration of the specific cellular events of implantation in their sequence helps to avoid the siren song simplicity of single mechanisms, substances or stimuli as the causative agent of implantation.