We use a set of empirical and analytical tools to conduct parallel analyses of involuntary part-time work and unemployment in the U.S. labor market. In the empirical analysis, we document that the similar cyclical behavior of involuntary part-time work and unemployment masks major differences in the underlying dynamics. Unlike unemployment, variations in involuntary part-time work are mostly explained by its interaction with full-time employment, and since the Great Recession employed workers are at a greater risk of working part-time involuntarily than being unemployed. In the theoretical analysis, we show that the higher probability of regaining full-time employment is key to distinguish involuntary part-time work from unemployment from a worker's perspective. We also quantify the welfare costs of cyclical fluctuations in involuntary part-time work, and the amplification of these costs arising from the elevated levels of involuntary part-time work observed since the Great Recession.