As indicated in the title, this thesis examines access to justice in multilevel trade regulation with a focus on Brazil, the "Common Market of the South" (MERCOSUR) and the World Trade Organization (WTO). Given that there is a direct link between the MERCOSUR and the European Union (EU), because the former is in several aspects comparable to the European Economic Community (EEC) and even the European Communities (EC), the research comprises a comparative legal analysis among four legal systems: (1) the Brazilian, (2) the MERCOSUR, (3) the EU and (4) the WTO. In order to achieve this goal, it employs legal texts, case law and scholarship in different languages (i.e., English, German, Portuguese and Spanish) and from different jurisdictions. While on the one hand it endeavours to explain the problems of access to justice in multilevel trade regulation and how they may be managed, on the other hand it intends to identify what access to justice and rule of law mean in the context of conflicts between the Brazilian, MERCOSUR and WTO jurisdictions. The thesis is structured into six main chapters, as follows: (I) the Constitutional Dimension of Access to Justice, (II) the Legislative Dimension of Access to Justice, (III) the Brazilian Dimension of Access to Justice, (IV) the MERCOSUR Dimension of Access to Justice, (V) the WTO Dimension of Access to Justice and (VI) the Final Conclusions. It begins by clarifying the author's personal understanding of what access to justice is. Then, it argues that the background of multilevel judicial protection is essentially formed by the proliferation of international courts and tribunals in general and, specifically to trade, the proliferation of regional trade agreements and free trade agreements, which very often include some form of dispute settlement system. Accordingly, divergent or even conflicting rulings regarding the same dispute and/or the same or similar legal issue are possible. The research undertaken extends, therefore, Mauro Cappelletti's world famous comparative legal research on access to justice. Furthermore, by expanding the work of the Italian jurist into the field of international economic law and establishing links to EU law, human rights, constitutional law, constitutionalism and rule of law, among others, this thesis also argues that constitutionalism is an effective mechanism for limiting abuses of power and protecting human rights, and is a way of connecting diverse regimes.