The young adulthood years are demographically dense. Dr. Ronald Rindfuss made this claim when he was Population Association of America (PAA) president in 1991 (Rindfuss 1991), and this conclusion holds today. I offer both an update of his work by including Millennials and a new view on young adulthood by focusing on an increasingly common experience: cohabitation. I believe we need to move away from our marriage-centric lens of young adulthood and embrace the complexity that cohabitation offers. The cohabitation boom is continuing with no evidence of a slowdown. Young adults are experiencing complex relationship biographies, and social science research is struggling to keep pace. Increasingly, there is a decoupling of cohabitation and marriage, suggesting new ways of framing our understanding of relationships in young adulthood. As a field, we can do better to ensure that our theories, methods, and data collections better reflect the new relationship reality faced by young adults.