Flowers, the reproductive organs of angiosperms (flowering plants), are more varied than the equivalent structures of any other group of organisms. Because of this variation, interest in plant sexual diversity has a long and venerable history in biology. Linnaeus used variation in sexual structures of plants as the basis for his classification, Darwin wrote three volumes on plant reproductive biology and Fisher provided the population-genetic principles for understanding the evolution of plant mating systems. However, despite this attention, it is still not obvious why flowering plants have evolved such extraordinarily diverse means to achieve only one primary function — mating success. A central problem for evolutionary biologists is therefore to understand the selective forces that are responsible for the origins and maintenance of sexual diversity in flowering plants . Here, I review recent advances in our understanding of the evolution and functional significance of sexual diversity in flowering plants. Several developments make it an opportune time to explore this topic. First, a rich theoretical framework and new advances in genetics and molecular biology have meant that sexual systems have become a unique testing ground for both phenotypic and genetic models of natural selection1–3,as well as for phylogenetic analysis and comparative biology4–6. Second, because plants are so amenable to experimental manipulation, they can be exploited as model systems for testing adaptive hypotheses on the function of sexual traits7–9. Last, the recent unification of pollination biology and mating system studies10, two areas of reproductive biology that remained separate for most of the past century, has led to new experimental findings on the mechanisms that drive sexual diversification in plants.