This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book contains a series of vivid photographs to illustrate the realities of some of the numerous ongoing environmental justice struggles across the world. It starts with a broad historical overview of environmental justice by Esme G. Murdock. The book explores some of the most common theories and concepts used by scholars to define the “justice” of environmental justice. It focuses on what have arguably become the four dominant theories of environmental justice: distribution, participation and procedure, recognition, and capabilities. The book looks at different issues that have been the focus of environmental justice struggles and offers insight into a perspective emerging from the Global South: the decolonial environmental justice approach. It discusses the dominant concept of sustainability and explores its relation to environmental justice.