This thesis deals with the integration of firms into export markets when trade barriers decrease. It consists in three chapters.The first chapter focuses on how cross-industry differences in factor intensities and within-industry differences in firm productivity shape the response of the extensive (the decision to export) and the intensive (the exported volumes) margin of export. The context of the analysis is the entry of Turkey into the European Customs Union in 1996. Results suggest that the extensive margin reacted more in labor-intensive sectors.In the second chapter we use a gravity approach to analyze how the decrease in tariffs promoted during the '90s by the Uruguay Round multilateral agreement affected trade margins for French fir...