Anisogamy is a form of sexual reproduction that involves the union or fusion of two gametes that differ in size and/or form. The smaller gamete is male, a microgamete or sperm cell, whereas the larger gamete is female, a larger macrogamete or typically an egg cell. Anisogamy is predominant among multicellular organisms. In both plants and animals, gamete size difference is the fundamental difference between females and males.

Anisogamy most likely evolved from isogamy. Since the biological definition of male and female is based on gamete size, the evolution of anisogamy is viewed as the evolutionary origin of male and female sexes. Anisogamy is an outcome of both natural selection and sexual selection, and led the sexes to evolve different primary and secondary sex characteristics including sex differences in behavior.

Geoff Parker, Robin Baker, and Vic Smith were the first to provide a mathematical model for the evolution of anisogamy that was consistent with modern evolutionary theory. Their theory was widely accepted but there are alternative hypotheses about the evolution of anisogamy.

Anisogamy (the opposite of isogamy) comes from the ancient Greek negative prefix a(n)- (alpha privative), the Greek adjective isos (meaning equal) and the Greek verb gameo (meaning to have sex/to reproduce), eventually meaning "non-equal reproduction" obviously referring to the enormous differences between male and female gametes in size and abilities. The first known use of the term "anisogamous" was in the year 1891.

Anisogamy is the form of sexual reproduction that involves the union or fusion of two gametes which differ in size and/or form. The smaller gamete is considered to be male (a sperm cell), whereas the larger gamete is regarded as female (typically an egg cell, if non-motile).

There are several types of anisogamy. Both gametes may be flagellated and therefore motile. Alternatively, as in flowering plants, conifers and gnetophytes, neither of the gametes are flagellated. In these groups, the male gametes are non-motile cells within pollen grains, and are delivered to the egg cells by means of pollen tubes. In the red alga Polysiphonia, non-motile eggs are fertilized by non-motile sperm.

The form of anisogamy that occurs in animals, including humans, is oogamy, where a large, non-motile egg (ovum) is fertilized by a small, motile sperm (spermatozoon). The egg is optimized for longevity, whereas the small sperm is optimized for motility and speed. The size and resources of the egg cell allow for the production of pheromones, which attract the swimming sperm cells.

Anisogamy is a core element of sexual dimorphism that helps to explain phenotypic differences between sexes. Researchers estimate that over 99.99% of eukaryotes reproduce sexually. Most do so by way of male and female sexes, both of which are optimized for reproductive potential. Due to their differently sized and shaped gametes, both males and females have developed physiological and behavioral differences that optimize the individual's fecundity. Since most egg laying females typically must bear the offspring and have a more limited reproductive cycle, this typically makes females a limiting factor in the reproductive success rate of males in a species. This process is also true for females selecting males, and assuming that males and females are selecting for different traits in partners, would result in phenotypic differences between the sexes over many generations. This hypothesis, known as the Bateman's Principle, is used to understand the evolutionary pressures put on males and females due to anisogamy. Although this assumption has criticism, it is a generally accepted model for sexual selection within anisogamous species. The selection for different traits depending on sex within the same species is known as sex-specific selection, and accounts for the differing phenotypes found between the sexes of the same species. This sex-specific selection between sexes over time also leads to the development of secondary sex characteristics, which assist males and females in reproductive success.