With the recognition afforded by evolutionary science that female infidelity was a recurrent feature of our evolutionary past has come the development of a new area of study within human mating: sperm competition. A form of male-male postcopulatory competition, sperm competition occurs when the sperm of two or more males concurrently occupy the reproductive tract of a female and compete to fertilize her ova. Just as males must compete for mates, if two or more males have copulated with a female within a sufficiently short period of time, the sperm from different males will compete for fertilizations. In the 2 decades since Smith (1984) first argued that sperm competition occurs in humans, this theory has been enriched with new ideas and discoveries. We review the recent theoretical and empirical work on human sperm competition, identify limitations and challenges of the research, and highlight important directions for future research.