This paper documents that much of the decline in labor force participation of U.S. prime age men comes from “in-and-outs”—who I define as men who temporarily leave the labor force. Individuals moving in and out of the labor force have been an understudied margin of labor supply but account for 20–40% of the decline in participation between 1984 and 2011. In-and-outs are distinct from unemployed individuals, experiencing no loss of future income as a result of their time out of the labor force, and represent a distinct margin of labor supply from long-term labor force dropouts. Examining explanations for the rise of in-and-outs, I find little evidence to suggest that changes in labor demand are responsible.