How do non-state actors matter in international relations? This volume recognizes three types of non-state actor: non-governmental organizations (NGOs), intergovernmental organizations (IGOs) and transnational corporations. It illustrates how they play roles alongside nation-states and are interrelated in matters of international regulation and coordination. After an introductory part on current qualitative and quantitative sources, this comprehensive collection of state-of-the-art essays is comprised of four main thematic parts: Part II examines actors other than governments, such as transnational religious actors, business representatives and experts, and also parliamentarians and agencies set up by IGOs. Part III studies the perceptions and understandings in political philosophy, international law and international relations theory. It questions concepts used (civil society, NGO, governance) and covers the limitations to be kept in mind. Part IV analyses the nature and impact of non-state actors. Chapters discuss processes within international bureaucracies (diplomacy, dynamism, bureaucratic power, contribution to democracy) and the quintessence of deliberation and decision making within NGOs and IGOs and of implementation, accountability and dispute settlement. Part V studies specific worlds of non-state actors: humanitarian aid, human rights, security, the North-South divide, health, trade and environment. Accessible and articulately written, The Ashgate Research Companion to Non-State Actors is aimed at a wide readership of scholars and practitioners in international relations.