Elevation of sperm Ca2+ seems to be responsible for an asymmetric form of motility called hyperactivation, which is first seen near the time of fertilization. CatSper family proteins are putative cation channels expressed exclusively in the membranes of the sperm flagellum. Hyperactivation requires CatSper proteins, which presumably serve as the route of entry for Ca2+ that operates directly on the flagellar axoneme to increase waveform asymmetry. In this article, the structure of mouse CatSper, the role it plays in sperm hyperactivation and the unsolved problems are described.