Why do growth and net exit rates of establishments decline with size? What determines the size distribution of establishments? This paper presents a theory of establishment dynamics that simultaneously rationalizes the basic facts on economy-wide establishment growth, net exit, and size distributions. The theory emphasizes the accumulation of industry-specific human capital in response to industry-specific productivity shocks. It predicts that establishment growth and net exit rates should decline faster with size and that the establishment size distribution should have thinner tails in sectors that use human capital less intensively or physical capital more intensively. In line with the theory, the data show substantial sectoral heterogeneity in U.S. establishment size dynamics and distributions, which is well explained by variation in physical capital intensity.