Book excerpt: This paper reviews the empirical evidence on firm heterogeneity in international trade. A first wave of empirical findings from micro data on plants and firms proposed challenges for existing models of international trade and inspired the development of new theories emphasizing firm heterogeneity. Subsequent empirical research has examined additional predictions of these theories and explored other dimensions of the data not originally captured by them. These other dimensions include multi-product firms, offshoring, intra-firm trade and firm export market dynamics.