Sexual reproduction provides genetic exchange in eukaryotic organisms as varied as animals, fungi and plants. Given its pervasiveness, sex is thought to have evolved once, probably associated with or soon after the origin of eukaryotic organisms. The fundamental principles of sex are preserved, including ploidy changes, the process of meiosis and formation of gametes, recognition of mate and cell to cell fusion leading to the creation of a zygote. Although the basic tenants are shared, sex determination and sexual reproduction occur in myriad forms throughout nature, including outbreeding systems with two mating types or sexes, unisexual selfing, and even examples in which organisms switch mating type. As provided with diverse and robust genetic models, kingdom fungi offer insights into the molecular nature of sex, sexual condition, and evolution to precede our understanding of sexual reproduction and its impact throughout the eukaryotic tree of life.