This chapter explores key definitions and characteristics of environmental movements by drawing on insights from Social Movements Studies. It examines the actors, actions, and multilateral context of environmental politics, as well as the emergence and influence of such movements. The chapter also introduces Green Politics within International Relations and maps the geographies of environmental activism. Emotions are addressed as central to social movements, particularly how they trigger environmental activism through fear, hope, and trauma experienced by people and nature. This emotional landscape is linked to mechanisms of action. The chapter concludes by presenting the core research questions guiding the book and offers an outline detailing the themes and content of each chapter, providing readers with a roadmap for the discussions that follow.