Agricultural production continues to shift to larger farms in the U.S. I show that the shift is persistent over time, large, and ubiquitous across commodities. I review theories of farm size, and classify three channels for analysis: 1) scale effects, through technological economies and managerial diseconomies; 2) the roles of relative factor prices and factor shares; and 3) policy and institutions. Finally, I evaluate the empirical evidence on the forces driving structural change, distinguishing between crops and livestock because of important differences in the role of scale economies and coordination, and I offer some directions for the future.