Ecological character displacement is phenotypic evolution wrought or maintained by resource competition between species. By resource competition, I mean the negative impact of one species (or individual) on another arising from depletion of shared resources. Character evolution driven by other mutually harmful interactions, such as intraguild predation or behavioral interference, is not included in the current definition of character displacement but perhaps should be in future. For the purposes of this chapter, however, character displacement is synonymous with the coevolution of resource competitors. The idea that competition between species has a significant impact on character evolution has a lively history. Prior to about 20 years ago, competition was seen as one of the major factors responsible for the evolution of species differences, particularly in traits affecting resource exploitation (e.g., body size, beak shape). The idea is seen again and again in the early literature not because it was rigorously established but because it so readily accounted for observed patterns of species differences in nature. Support for the idea began to slip soon afterward, however, as alternative hypotheses were developed and as it became clear that the quality of most of the available evidence was poor. More recently, the idea has become respectable again as evidence from several systems has become more solid. My goal in this chapter is to present an overview of some of this evidence and how it has affected our understanding of the process. I begin with a brief historical sketch of character displacement and the expectations from theory. I then present a few of the highlights emerging from observational studies of patterns suggesting character displacement, their limitations, and their implications. I follow with an overview of recent experimental work that complements studies of pattern but goes beyond them by testing novel predictions of character displacement hypotheses. I end with suggestions about where the most significant future discoveries lie. The history of ideas on competition and character divergence begins with Darwin (1859), who regarded interspecific competition for resources as a fundamental and ubiquitous agent of divergent natural selection: “Natural selection . . . leads to divergence of character, for more living beings can be supported on the same area the more they diverge in structure, habitats, and constitution”.