In many flowering plants, intraspecific barriers to self-fertilization ensure outbreeding by interrupting the path of pollen tube growth before egg-sperm interaction. These self-sterility or self-incompatibility (SI) systems are unique among recognition systems, in that they are based on the recognition and rejection of self, rather than non-self. Consistent with multiple evolutionary origins of SI, different plant families use distinct mechanisms for pollen recognition and inhibition. These mechanisms range from the arrest of pollen tube emergence by specific interactions between surface receptors and ligands and activation of intracellular signaling cascades, to the destruction of pollen tubes by the cytotoxic activity of RNases or by induction of a programmed cell death pathway. These mechanistic differences notwithstanding, all plant SI systems analyzed to date share a high degree of recognition specificity, effected by a large number of highly diverged, genetically linked and co-evolving genes, with immunity and other systems that discriminate between self and non-self.