The stigma and style are the two differentiated organs of the flower that carry out the unique function of gamete screening and selection during pollination. These floral organs undergo subtle morphological and physiological changes from the time they are carved out on the receptacle, and by the end of their growth phase they develop a set of unusual features for the specified functions. It was pointed out at some length in Chapter 9 that the current models for pollen–stigma–style interactions during self-incompatibility predict the existence of a precise control system involving expression of the S-locus both in the pollen grains and in the stigmatic papillae or the style. Supporting this prediction, S-gene products from the pistil were identified as specific glycoproteins, although thus far no pollen component has been found. Based on this model, the widely accepted view is that self-incompatibility is due to the composite reaction of two somewhat similar, if not identical, molecules. The past two decades have witnessed major advances in the isolation and characterization of S-locus genes of the stigma–style complex and of their protein products which mediate in self-incompatibility reactions in some model systems. Included in the list of model systems are plants that display either gametophytic and sporophytic types of self-incompatibility. This chapter will deal with the fundamental components of S-locus genes and their protein products.