Fertilization is a result of complex molecular interactions between sperm and egg and a complete understanding of these processes is necessary to identify potential sites for contraceptive development as well as specific molecular lesions leading to infertility. Our laboratory is currently employing advanced imaging technologies along with biochemical and biophysical methodologies in order to determine both the molecules involved in fertilization as well as the precise sequence of molecular events ultimately leading to the fusion of mammalian sperm and egg. In particular we have been focusing on three specific aspects of sperm egg interaction: The characterization of egg-associated proteins with complementary receptors on the sperm surface. The dynamics of the sperm plasma membrane during fertilization. The initiation and characterization of signal transduction pathways leading to the exocytosis of the acrosomal vesicle from sperm. Using a number of video enhanced microscopic techniques such as fluorescence recovery after photobleaching (FRAP) and fluorescence resonance energy transfer (FRET) we are able to observe these molecular events at the level of the single cell. In addition, we have begun to use these techniques to follow the activation of other cell types and to follow the expression of genes during embryonic development.