In the course of plant domestication and improvement, many genetic changes have been wrought by selection and other techniques to transform wildlings to biotypes that are adapted for cultivation and useful productivity. During this process the mating system of various cultigens has been altered either intentionally or unintentionally. In investigations of the tomato species we have encountered much evidence of changes in mating systems both intra- and interspecifically. This chapter will discuss primarily modifications during and prior to domestication of the cultivated tomato (Lycopersicon esculentum) and other crop species. Various aspects of this subject have been treated by Frankel and Galun (1977), de Nettancourt (1977), Simmonds (1979), and others.