The fraction of the US workforce identified as involuntary part-time workers rose to new highs during the US Great Recession and came down only slowly in its aftermath. We assess the determinants of involuntary part-time work using an empirical framework that accounts for business cycle effects and persistent structural features of the labor market. We conduct regression analyses using state-level panel data for the years 2003–16. The results indicate that structural factors, notably shifts in the industry composition of employment, have held the incidence of involuntary part-time work slightly more than 1 percentage point above its prerecession level.