Contemporary race and immigration scholars often rely on historical analogies to help them analyze America’s current and future color lines. If European immigrants became white, they claim, perhaps today’s immigrants can as well. But too often these scholars ignore ongoing debates in the historical literature about America’s past racial boundaries. Meanwhile, the historical literature is itself needlessly muddled. In order to address these problems, the authors borrow concepts from the social science literature on boundaries to systematically compare the experiences of blacks, Mexicans, and southern and eastern Europeans (SEEs) in the first half of the 20th century. Their findings challenge whiteness historiography; caution against making broad claims about the reinvention, blurring, or shifting of America’s color lines; and suggest that the Mexican story might have more to teach us about these current and future lines than the SEE one.