Research on the disparity in common mental health problems borne by sexual minority individuals has entered a stage of increasing theoretical complexity. Indeed, such a substantial disparity is likely not determined by a singular cause and therefore warrants diverse etiological perspectives tested with increasingly rigorous methodologies. The research landscape is made even more complex by the constant and rapid shift in the ways in which sexual minority people understand and characterize their own identities and experiences. This review introduces readers to this complexity by summarizing the historical legacy of research on the sexual orientation disparity in mental health, describing five contemporary theoretical explanations for this disparity and their supporting evidence, and suggesting theoretically informed interventions for reducing this disparity. Last, we offer an agenda for future research to accurately model the complexity of the pathways and solutions to the disproportionately poorer mental health of sexual minority populations.