My thesis is composed by three chapters that study the important role that firms play in mediating imports and exports between countries, and how their decisions and heterogeneity can influence the behavior of macroeconomic outcomes. The first chapter, explores the role of demand uncertainty and learning in explaining the low survival rates and sales of new exporters using a manufacturing survey for Colombian firms. In the second chapter, using a transaction-level dataset for Colombian exporters, I document novel facts about the post-entry dynamics of firm performance and prices in international markets, and study their connection with the transmission of exchange rate shocks into firm-level prices. To rationalize these facts, I propose a t...