A major goal of evolutionary biology is to understand how diverging populations become species. The evolution of reproductive isolation (RI) halts the genomic homogenization caused by gene flow and recombination, and enables differentiation and local adaptations to become fixed between newly forming species. Selection can favor the strengthening of RI through a process termed reinforcement. Reinforcement occurs when selection favors traits that decrease mating between two incipient species in response to costly mating or the production of maladapted hybrids. Although this process has been investigated more frequently in animals, there is also evidence of reinforcement in plants. There are three strategies for the investigation of the process of reinforcement: case studies of species or diverging taxa; experimental evolution studies; and comparative studies. Here, I discuss how all three strategies find evidence consistent with reinforcement occurring in plants. I focus largely on case studies, and use research on Phlox drummondii to illustrate the importance of testing alternative hypotheses. Although the existing evidence suggests that reinforcement can occur, further investigations, particularly using large-scale comparative studies, are needed to determine the importance of reinforcement in plant speciation.