In recent decades, the world trading system has witnessed the proliferation of both, preferential trade agreements (PTAs) and Non-Tariff Measures (NTMs). This dissertation contributes to our understanding of the impact of the two phenomena by: (i) examining the effect of PTAs on the multilateral system, and (ii) assessing the impact of Sanitary and Phytosanitary (SPS) measures on market entry and international trade flows. The first two chapters examine the questions: what is the impact of preferential trade liberalization on external protection? The first chapter provides evidence that the impact of PTAs is heterogeneous across countries and argues that the effect is mainly driven by tariff revenue considerations. The second chapter explores the role played by the enhanced market opportunities conferred by regional agreements on external protection. The last chapter uses restrictive product standards to assess the SPS impact on the extensive and the intensive margin of trade in agricultural and food products.