This chapter examines homomorphic self-incompatibility. to escape inbreeding, angiosperms have developed well-organized systems such as dichogamy and self-incompatibility. Plants having self-incompatibility cannot achieve fertilization with the sperm of self-pollen, although they produce functional male and female organs. Self-incompatibility involves a recognition reaction between self and non-self in higher plants. in higher plants, recognition reactions are only known in the pollination system, involving self- and interspecific incompatibility, and in plant–pathogen interactions. The homomorphic type of self-incompatibility includes sporophytic and gametophytic systems, which are defined by the action of S genes. in the sporophytic system, the behavior of pollen tubes is determined by the phenotype of the sporophyte, by which the pollen is produced. The self-incompatibility system is composed of three stages: (1) germination of the pollen grain, (2) ability of the pollen tube to penetrate the cuticle of the stigmatic papilla, and (3) interaction of substances secreted by the pollen tube with products of the stigmatic cytoplasm. A signal transduction system appears to be involved in sporophytic incompatibility, as SRK (S-receptor kinase) genes are found. Such genes have not been found to be involved in gametophytic incompatibility.