This paper discusses the role of firms in international trade, highlighting the differences between trading and non-trading firms and how these differences challenge traditional trade models. It introduces new stylized facts about firms and trade using transaction-level U.S. trade data, showing that the extensive margins of trade—number of products and countries traded—are central to understanding the role of distance in dampening trade flows. The paper also discusses empirical challenges to old and new trade theories, emphasizing the importance of firm heterogeneity in explaining trade patterns. It highlights that exporting is a rare activity, with only a small percentage of firms engaging in it. Exporters are found to be larger, more productive, and more skill- and capital-intensive than non-exporters. The paper also discusses how trade liberalization can lead to productivity growth through reallocation of resources among firms. New trade theories, such as those based on heterogeneous-firm models, provide a more comprehensive explanation of trade patterns by incorporating firm heterogeneity. The paper also discusses the concentration of trade among a small number of firms and the importance of multi-product exporters in overall trade. It concludes that the extensive margins of trade, particularly the number of products and countries traded, are crucial in understanding trade patterns and that trade liberalization can lead to significant productivity gains through reallocation of resources among firms.This paper discusses the role of firms in international trade, highlighting the differences between trading and non-trading firms and how these differences challenge traditional trade models. It introduces new stylized facts about firms and trade using transaction-level U.S. trade data, showing that the extensive margins of trade—number of products and countries traded—are central to understanding the role of distance in dampening trade flows. The paper also discusses empirical challenges to old and new trade theories, emphasizing the importance of firm heterogeneity in explaining trade patterns. It highlights that exporting is a rare activity, with only a small percentage of firms engaging in it. Exporters are found to be larger, more productive, and more skill- and capital-intensive than non-exporters. The paper also discusses how trade liberalization can lead to productivity growth through reallocation of resources among firms. New trade theories, such as those based on heterogeneous-firm models, provide a more comprehensive explanation of trade patterns by incorporating firm heterogeneity. The paper also discusses the concentration of trade among a small number of firms and the importance of multi-product exporters in overall trade. It concludes that the extensive margins of trade, particularly the number of products and countries traded, are crucial in understanding trade patterns and that trade liberalization can lead to significant productivity gains through reallocation of resources among firms.