Using highly disaggregated transaction-level trade data, we document the importance of new firm level trade partner relationships and the addition of new products to existing relationships in driving long-run import flows. Moreover, we find that these margins are sensitive to movements in the exchange rate. We rationalize these findings in a model of international trade with endogenous matching between heterogenous importers and exporters. Simulations of the model highlight a new channel through which exchange rate movements can affect trade—through the short-run formation of new trade relationships and the range of products traded within relationships—which can impact long-run flows.