Sperm cells must undergo a complex maturation process after ejaculation to be able to fertilize an egg. One component of this maturation is hyperpolarization of the membrane potential to a more negative value. The ion channel responsible for this hyperpolarization, SLO3, was first cloned in 1998, and since then much progress has been made to determine how the channel is regulated and how its function intertwines with various signaling pathways involved in sperm maturation. Although SLO3 was originally thought to be present only in the sperm of mammals, recent evidence suggests that a primordial form of the gene is more widely expressed in some fish species. As with many reproductive genes, SLO3 is rapidly evolving with low conservation between closely related species and different regulatory and pharmacological profiles. Despite these differences, SLO3 appears to have a conserved role in regulating sperm membrane potential and driving large changes in response to stimuli. As with the differences in regulation of the channel, the effect of this hyperpolarization of the membrane potential may vary among mammalian species. Recent discoveries have elucidated the role of SLO3 in these processes in human sperm and provided tools to target the channel to affect human fertility.