This chapter introduces taxonomy of non-governmental organizations (NGO) in Brussels, explain the importance of NGOs in the political system of the European Union (EU), illustrate the functions of NGOs in the integration process and discuss democratic challenges of the system of interest representation in the EU. Since the Treaty of Rome came into force in 1958, NGOs have played an important role in the process of European integration. The EU is primarily an economic and legal community aiming to realize a common market that includes a single currency and fundamental market freedoms. The European Commission has acknowledged the problem and, with its White Paper on European Governance, initiated a consultation regime that bestowed on NGOs a more prominent role in the process of EU policy-making by opening venues of interest representation. Most of the NGOs represented in Brussels are established as voluntary associations under Belgian law.