Declining marriage rates suggest a growing fraction of millennials will remain unmarried through age 40. In this brief, we use data from the American Community Survey to estimate age-specific marriage rates and project the percentage of millennials who will marry by age 40 in different scenarios. We find that the percentage of millennials marrying by age 40 will fall lower than for any previous generation of Americans, even in a scenario where marriage rates recover considerably. Moreover, marriage patterns will continue to diverge by education and race, increasing the divides between mostly married "haves" and increasingly single "have-nots". Two appendices are included: (1) Details of the Methodology; and (2) Marriage Trends in the Foreign-Born Population.