S-RNase-based self-incompatibility (SI) occurs in the Solanaceae, Rosaceae and Plantaginaceae, with S-RNases determining the specificity of pollen rejection in the pistil and S-locus F-box proteins fulfilling this function in pollen. McClure et al. introduce the genetics of SI and briefly describe the characteristics of S-RNases and pollen F-box proteins. Two alternative mechanisms have been proposed whereby compatibility is explained either as a result of degradation of non-self S-RNase or by its compartmentalization so that it does not have access to the pollen tube cytoplasm. These models are not necessarily mutually exclusive, but each makes different predictions about whether pollen compatibility or incompatibility is the default.