We construct new monthly time series of U.S. labor market stocks and flows from 1976 onwards. These data reveal an upward secular trend in turnover between full-time and part-time employment, and a large cyclical component chiefly explained by fluctuations in involuntary part-time work. Both short-run and long-run reallocations occur mostly without an intervening spell of non-employment, and therefore cannot be uncovered without splitting employment into finer categories. We emphasize the importance of our findings for several active debates, such as the slowdown in U.S. labor-market dynamism, changes in job stability and security, and the assessment of labor-market slack.