Tension between the pursuit of trade liberalization and the protection and exercise of national regulatory autonomy is an issue that has arisen in both the European Union (EU) and the World Trade Organization (WTO). While both legal orders provide for exceptions to the fundamental rules concerning free movement and access to markets, the interpretation and application of these exceptions represent a challenge. The fundamental question is how to distinguish between unlawful barriers to trade and legitimate national regulation. This paper presents an analysis of the approaches applied to this question by the EU and WTO, in the areas of free movement of goods and services, from a comparative perspective. It explores, in particular, the implications of the market access – non-discrimination dichotomy, and focuses upon the role of ‘proportionality’ review in each legal order.