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Publisher Summary Dioecism has arisen independently in different plant families and plant genera. In the majority of cases, the evolution of dioecism has apparently taken place on the species level; in some cases, on the subgeneric level (rumex) or generic level (humulus). Few families (salicaceae) comprise only dioecious species. Dioecious plants offer, in many cases, better tools for studying the genetics of sex determination than dioecious animals. First, the fact that dioecious plants have arisen independently gives an opportunity to study the different ways in which dioecism may become established. Second, in plants the step from dioecism to bisexuality is often a short one and in most cases, dioecism is not clear-cut. In many dioecious species (e.g. mercurialis), bisexual types are found in nature often with a rather high frequency. Such bisexual individuals of normally dioecious plant species are almost always fertile and can be genetically studied, whereas similar bisexual animals are sterile intersexes. In plants it is possible to follow both the evolution of dioecism from hermaphroditism (or monoecism) and the reverse process.