This paper uses data from the Census Bureau's Longitudinal Research Database to describe the dynamics of geographic concentration in U.S. manufacturing industries. Agglomeration results from a combination of the mean reversion and randomness in the growth of state-industry employment. Although industries' agglomeration levels have declined only slightly over the last quarter century, we find a great deal of movement for many geographically concentrated industries. We decompose aggregate concentration changes into portions attributable to plant births, expansions, contractions, and closures. We find that the location choices of new firms play a deagglomerating role, whereas plant closures have tended to reinforce agglomeration.