SYNOPSIS. Free-spawning marine invertebrates face the challenge of ensuring that gametes of the same species come into contact, recognize, bind to and fuse with one another once they have been released by the adults. Coordinated spawning, chemoattraction and specific cell-cell recognition events help to overcome this challenge. One marine invertebrate, the sea urchin, has served as a model system for the study of gamete recognition and fertilization for over 100 years. Recent biochemical and molecular advances in this area have begun to address the questions that have been raised by the results of elegant physiological observations. The picture of fertilization that is emerging is characterized by highly specific cell-cell interactions between proteins on the surfaces of the gametes. These proteins then mediate the binding and subsequent events that lead to activation of the egg and delivery of the male genetic material. Because of these recent insights, the sea urchin egg is in a position to provide answers to one of the central debates in developmental biology—the mechanism of egg activation. Does the sperm deliver an activating factor? Does sperm binding trigger a receptor-mediated signal? Or is the mechanism a complex combination? With the tools and knowledge gained from the study of sea urchin fertilization, testing of these hypotheses should be feasible in the near future.