Firms are heterogeneous, even within narrowly defined sectors. This article surveys the relevant theoretical and empirical literature on firm heterogeneity and external trade. By innovatively exploiting rich crosscountry micro-aggregated data sourced from the ECB Competitiveness Research Network (CompNet), this study investigates the main implications of firm heterogeneity for trade by EU countries, presenting a set of stylized facts. On the one hand, exporting firms are larger, more productive and pay higher wages than non-exporting firms. Indeed, only these firms are able to bear export costs arising from various factors, such as tariff and non-tariff trade barriers, the quality of the legal system or access to finance. Hence, only few enterprises actually export and the intensity of aggregate export concentration within a few large firms varies across countries and sectors. On the other hand, engaging in trade boosts individual firms' productivity growth via a number of channels and enhances allocative efficiency across firms, in turn increasing aggregate productivity growth. One of the main standard determinants of export growth, namely changes in the real effective exchange rate, impacts aggregate performance differently across countries depending on sectoral composition and on firm characteristics within a given sector.