Sex determination is the process by which an organisms biological sex is determined. In most organisms that reproduce sexually, there are two common sexes, male and female, and a few less common intersex variations. The process of sex determination can be influenced by genetic and environmental factors. In mammals, primary sex determination is strictly chromosomal and is not usually influenced by environmental factors. In humans, sex determination is determined by the chromosome that the male parent contributes. Males have XY sex chromosomes while females have XX sex chromosomes.

In some species, sex determination is genetic, where males and females have different alleles or even different genes that specify their sexual morphology. In animals, this is often accompanied by chromosomal differences, generally through combinations of XY, ZW, XO, ZO chromosomes, or haplodiploidy. The sexual differentiation is generally triggered by a main gene (a "sex locus"), with a multitude of other genes following in a domino effect.

Sex determination involves the development of individuals with characteristics that allow them to be identified as males, females, or in some cases, hermaphrodites. In certain species, differences in sexual characteristics can be very small, while in others, the phenotypic differences between the sexes can be quite significant.

The process of sex determination begins after fertilization, where male and female germ cells fuse to form a zygote, or a single-celled, fertilized egg. Male germ cells are sperm cells and female germ cells are egg cells. When the egg and sperm cells fuse, the zygote divides into multiple cells and later forms an embryo.