This study examines how relationship transitions affect subjective well-being (SWB) and how this effect changes over time. We used prospective data containing information about 18 years of young adults' lives (PSIN, N = 5, 514). SWB was measured with the Satisfaction with Life Scale. Within-person multilevel regression analyses showed that dating, unmarried cohabitation, and marriage had additional well-being enhancing effects. After entry into a union, well-being slowly decreased. A large SWB decrease was found after union dissolution, but through adaptation or repartnering well-being increased again. Well-being of never-married and never-cohabiting young adults decreased slowly over time. These effects were independent of parenthood and employment. Our results confirm expectations from the resources theory but contradict some assumptions of the set-point theory. © 2009 National Council on Family Relations.