In this paper we discuss the events leading to the recognition and fusion of sperms and eggs. In the course of evolution a glycoprotein coat, the vitelline coat, has differentiated around the egg. This coat carries the sperm receptors and is a product of the oocyte. The receptors are glycoprotein molecules; in most of the species studied fucose appears to be the most important component as regards receptor function. In most, though not in all, animals the interaction of the spermatozoa with the sperm receptors triggers the acrosome reaction. Some observations further suggest that the vitelline coat may also serve to repress the metabolism of the unfertilized egg.