One of the most exciting developments in the field of international trade in recent years has been the increased attention given to the heterogeneity of firms. Empirical evidence has demonstrated that only a minority of firms are involved with international trade, and that the firms that do trade are different in significant ways from other firms. Theoretical models have been developed that incorporate different types of firm, and trade economists have considered how the distribution of firms changes as economies become more open to international trade. The purpose of the current workshop is to outline and review this body of work.