This synthesis follows seminal works on diversity and equity in science education, including those by J. Oakes (1990), M. M. Atwater (1994), and S. Lynch (2000). It differs from these previous works in several important respects. First, it examines the relationship between students' science outcomes, broadly defined, and mechanisms or factors thought to affect outcomes. Particularly, it attempts to provide insights into why gaps in science outcomes among racial/ethnic, cultural, linguistic, and socioeconomic groups have persisted over the years, in the hope that such insights may point the way toward eliminating those gaps. Second, it is based predominantly on peer-reviewed journal articles and guided by other criteria whose aim is to ensure the methodological rigor of the research considered herein. Third, considering that a majority of the studies on diversity and equity in science education have been conducted and published since the mid-1990s, it offers the most current review of the literature. Finally, a major goal of this synthesis is to offer recommendations for a future research agenda.