We use transaction-level data to study changes in the concentration of US imports. Concentration has fallen in the typical industry, while it is stable by industry and origin country. The fall in concentration is driven by the extensive margin: the number of exporting firms has grown, and the...

We study how financial frictions affect firm-level heterogeneity and trade. We build a model where productivity differences across monopolistically competitive firms are endogenous and depend on investment decisions at the entry stage. By increasing entry costs, financial frictions lower the...

We use transaction-level US import data to compare firms from virtually all countries in the world competing in a single destination market. Guided by a simple theoretical framework, we decompose countries' market shares into the contribution of the number of firm-products, their average...