We document a number of novel facts on the role of importer, exporter, and product relation- ships in the determination of trade flows using very detailed transaction-level data on Canadian imports. Notably, we find that the import market is dominated by a few domestic importers and foreign exporters, and that the introduction of new products into pre-existing buyer-seller relationships accounts for nearly half of import flows. We rationalize these findings in a model of international trade with monopolistic competition and multi-product exporters. In particular, we show that imports respond to exchange rate movements mainly through adjusting the scope of products within existing buyer-seller relationships and through the pure intensive margin; the response through firm entry-and-exit into trade (pure extensive margin) is very small.