Book excerpt: This paper explores a newly available panel dataset merging balance sheet and international trade transaction data for Belgium. Both imports and exports appear to be highly concentrated among few firms and seem to have become more so over time. Focusing on manufacturing, we find that facts previously reported in the literature as applying only to exports actually apply to imports too. We note that the number of trading firms diminishes as the number of export destinations or import origins increases. The same is true if we consider the number of products traded. Our results generally point to a process of self-selection in both export and import markets. Also, the productivity advantage of exporters reported in the literature may be overstated because imports were not considered. We find that firms that both import and export are the most productive, followed, in descending order, by importers only, exporters only and non-traders. Our results also show the existence of fixed costs of imports, which appear to be of similar magnitude as those of exports.