The grasses are amongst the most important families of plants. The family includes major food crops and pastures and they dominate many natural ecosystems. Self-incompatibility (SI) is common in the grasses and is under the control of two unlinked loci, S and Z. Specification of SI in this family is gametophytic and is determined by the combination of S and Z alleles in the pollen grain. Available data suggests that the two-locus SI system is common to all self-incompatible grass species of the Pooideae and possibly all members of the Graminae. Genetic studies showed the presence of the S-Z system in the Triticeae, the Poeae and the Avenae. Linkage analyses have identified common markers linked to the S-gene in grass species from all three tribes. Molecular markers have confirmed the syntenous localisation of S and Z in several species. Although this SI system is complex relative to that in other families, the importance of this family and the detailed genetic and physical data emerging from many genomics programsmakes this an important group for more detailed studies.