We document that fluctuations in part-time employment play a major role in movements in hours per worker, especially during cyclical swings in the labor market. Building on this result, we propose a novel representation of the intensive margin based on a stock-flow framework. The evolution of part-time employment is predominantly explained by cyclical changes in transitions between full-time and part-time employment, which occur overwhelmingly at the same employer and entail large changes in individuals' working hours. We discuss implications for a large class of macroeconomic models that map individual decisions along the extensive/intensive margins onto aggregate labor market outcomes.