Genetic analyses, particularly in Arabidopsis, have led to the identification of mutants that define different steps of ovule ontogeny, pollen stigma interaction, pollen tube growth, and fertilization. Isolation of the genes defined by these mutations promises to lead to a molecular understanding of these processes. Mutants have also been obtained in which processes that are normally triggered by fertilization, such as endosperm formation and initiation of seed development, occur without fertilization. These mutants may illuminate apomixis, a process of seed development without fertilization extant in many plants.