This study is the first to comprehensively investigate international trade at the firm-level using Japan’s customs data for the 2014-2020 period. We first decompose international trade into the intensive and extensive margin and show that the intensive margin accounts for around 30% and 40% of the variation in partner country-specific exports and imports, respectively. We next find a substantial concentration of trading firms: in 2017, the top 10% of exporters accounted for 96.6% of all exports, while the top 10% of importers were responsible for 94.6% of all imports. Finally, we match the customs data with other firm-level datasets and estimate the performance premia of exporting firms. Our findings indicate that exporting firms outperform non-exporting firms in all aspects we consider: sales, value added, the number of employees, the capital-labor ratio, productivity, and wages. Interestingly, the exporter premia for value added, labor productivity, and total factor productivity decreased between 2014 and 2016 and then increased until 2019, whereas the exporter premium for the average wage steadily increased.