Using data from the Health and Retirement Study and Modeling Income in the Near Term, this paper examines the impact of the changing lives of women on Social Security replacement rates. Replacement rates have dropped sharply at both the household- and individual-level, and the decline will continue for future retirees. Decomposing the reasons for the overall decline shows that increases in the labor supply and earnings of women explain more than one-third of the change. In contrast, the impact of changing marital patterns is relatively small. Much of the remaining explanation rests with the increased Full Retirement Age and changing claiming behaviors.