This paper analyzes a puzzling aspect of retirement behavior known as "unretirement." Nearly 50 percent of retirees follow a nontraditional retirement path that involves partial retirement or unretirement, and at least 26 percent of retirees later unretire. I explore two possible explanations: (1) unretirement transitions result from failures in planning or financial shocks; and (2) unretirement transitions are anticipated prior to retirement, reflecting a more complex retirement process. I show that unretirement was anticipated for the vast majority of those returning to work, and is not a result of financial shocks, poor planning or low wealth accumulation. (Contains 2 figures, 8 tables, and 34 footnotes.)