Over the next generation or two, America’s older, largely white population will increasingly be replaced by today’s disproportionately poor minority children. All future growth will come from populations other than non-Hispanic whites as America moves toward a majority-minority society by 2043. This so-called Third Demographic Transition raises important implications about changing racial boundaries in the United States, that is, about the physical, economic, and sociocultural barriers that separate different racial and ethnic groups. America’s racial transformation may place upward demographic pressure on future poverty and inequality as today’s disproportionately poor and minority children grow into adult roles. Racial boundaries will be reshaped by the changing meaning of race and ethnicity, shifting patterns of racial segregation in neighborhoods and the workplace, newly integrating (or not) friendship networks, and changing rates of interracial marriage and childbearing. The empirical literature provides complicated lessons and offers few guarantees that growing racial diversity will lead to a corresponding breakdown in racial boundaries—that whites and minorities will increasingly share the same physical and social spaces or interact as coequals. How America’s older population of elected officials and taxpayers responds today to America’s increasingly diverse population will provide a window to the future, when today’s children successfully transition (or not) into productive adult roles. Racial and ethnic inclusion will be reshaped by changing ethnoracial inequality, which highlights the need to invest in children—now.