摘要：Despite the fact that importing and exporting are extremely rare firm
activities, economists generally devote little attention to the role of firms
when discussing international trade. This paper summarizes key differences
between trading and non-trading firms, demonstrates how these differences
present a challenge to standard trade models and shows how recent
“heterogeneous-firm” models of international trade address these challenges. We
then make use of transaction-level U.S. trade data to introduce a number of new
stylized facts about firms and trade. These facts reveal that the extensive
margins of trade – that is, the number of products firms trade as well as the
number of countries with which they trade – are central to understanding the
well-known role of distance in dampening aggregate trade flows
