The fraction of men working in the United States has declined consistently since the 1950s. This has contributed to slower labor force growth and resulted in considerable gaps between labor force participation in the U.S. and its industrialized peers. In this paper we examine the drivers of this trend, focusing specifically on prime-age men (aged 25–54). We compare non-participation rates across four generations – the Silent Generation, Baby Boomers, Generation X, and Millennials – and decompose generational gaps into “push” and “pull” factors that are intended to be descriptive, rather than causal, by design. We define pull factors as those that draw men out of the labor force such as schooling or caretaking. Push factors are those that limit labor market opportunities, such as skills mismatch or disability. Our findings suggest that both pull and push factors are important with the most notable being skills mismatch, caretaking responsibilities, and prolonged continuing education.