The ability of a plant to achieve its full reproductive potential depends upon the completion of an uninterrupted cycle of sexual and asexual processes. However, there are certain instances in which physiological and genetic barriers converge to prevent completion of the component processes of sporogenesis, gametogenesis, fertilization, and embryogenesis, and thus thwart seed set. In Chapter 5 it was seen that in a wide range of plants, the arrest of normal pollen development results in male sterility. It is now well established that the molecular and cellular organization of the pollen grain and stigma provides effective recognition systems at the time of pollination for screening suitable gametes for fertilization; this theme permeated most of Chapter 7. This chapter considers the precise genetic control of cell recognition that operates in many plants and enables an individual flower to distinguish between self- and nonself-pollen grains once they land on the stigma and begin to germinate. It is now clear that the sporophytic tissues of the flower play leading roles in both the recognition and the rejection of male gametes that reinforce the outbreeding potential of the species. Although the practical importance of these phenomena has not been fully exploited, they are of great developmental and functional significance in the reproductive biology of angiosperms.