This paper draws on work in the social construction of race and ethnicity to explain why race/ethnic divisions are so often axes of domination and why these divisions are central to social movements. Ethnic/racial groups are constructed in political processes that are intertwined with state formation and social movements. Processes of state formation create ethnic/racial groups, and define majorities and minorities, and create racial/ethnic structures of domination. Physical and social segregation in tandem with intergenerational inheritance creates network cliquing that reinforces group boundaries, group differences, and group interests. Movements by members of dominant racial/ethnic majorities differ from movements by members of subordinate racial/ethnic minorities in key ways including access to democratic processes for achieving group goals, experience of repression, need for allies, identity construction, processes of consciousness raising, and bases of mobilization. The "ethnic dimensions" matter for all social movements.