In the United States, racial-ethnic differences on tests of school readiness and academic achievement continue. A complete understanding of the origins of racial-ethnic achievement gaps is still lacking. This article describes social equity theory (SET), which proposes that racial-ethnic achievement gaps originate from two kinds of social process, direct and signal influences, that these two kinds of processes operate across developmental contexts, and that the kind of influence and the setting in which they are enacted change with age. Evidence supporting each of SET's key propositions is discussed in the context of a critical review of research on the Black-White achievement gap. Specific developmental hypotheses derived from SET are described, along with proposed standards of evidence for testing those hypotheses. (Contains 2 figures.)