Abstract: In this paper, we will introduce the effect of sperm competition—that is, the direct competition between the sperm of different males to fertilize a female’s eggs—on mating success and the evolution of mating systems. In some promiscuous and some polyandrous mating systems, males compete not only for access to mating opportunities with females, but directly for access to eggs. In these systems, competition occurs after a female has mated with numerous males. If females store sperm from numerous matings, sperm from different males may compete with one another over access to fertilizable eggs. When sperm competition exists, selection can operate directly on various attributes of sperm, such as sperm size and shape. If a male wins a physical fight with the opponents, this may provide him access to an estrous female that may lead to a mating opportunity, which may result to fertilization.