After a long decline, the Labor Force Participation Rate (LFPR) of older men in the United States leveled off in the 1980s, and began to increase in the late 1990s. We examine how changes in Social Security rules affected these trends. We attribute only a small portion of the decline from the 1960s-80s to the increasing generosity of Social Security over this period. Increases in the Full Retirement Age and the Delayed Retirement Credit explain one quarter to one half of the recent increase in the LFPR. Increasing educational attainment and increasing LFPR of married women also contributed to the recent rise. (Contains 10 figures, 6 tables, and 36 footnotes.)