Kin selection theory proposes that individuals value the reproductive success of relatives at a rate determined by their probability of shared alleles. The theory predicts when the interests of relatives are in accord and when they conflict. Though kin selection arguments have revolutionized the study of animal behavior, they have only recently been applied to plants. Kin selection has already been claimed to explain the formation of endosperm by double fertilization. This is the character that distinguishes angiosperms from gymnosperms. Plant life cycles involve interactions among kinds of relatives not encountered in animals. These interactions should be a fertile field for new applications of theory and the testing of ideas originally developed elsewhere.