Whole-cell voltage clamp of mammalian spermatozoa was first achieved in 2006. This technical advance, combined with genetic deletion strategies, makes unambiguous identification of sperm ion channel currents possible. This review summarizes the ion channel currents that have been directly measured in mammalian sperm, and their physiological roles in fertilization. The predominant currents are 1) a Ca 2+-selective current requiring expression of the 4 mCatSper genes, and 2) a delayed rectifier K+ current with properties most similar to mSlo3. Intracellular alkalinization activates both channels and induces hyperactivated motility.