We are facing hugely complex challenges - from climate change to world poverty, our problems are part of an inter-related web of social and natural systems. Human Ecology provides an approach to address these complex challenges, a way to understand them holistically and to start to manage them more effectively. This textbook, which has been road-tested and refined through over a decade of teaching and workshops, offers a coherent conceptual framework for Human Ecology - a clear approach for understanding the many systems we are part of and how we frame and understand the problems we face. By giving rigorous definitions it guides readers out of the current 'conceptual swamp' that hinders communication and collaboration - with a particular focus on terms such as "sustainability" and "cultural adaptation", that need generally agreed definitions before they can support clear communication. It also clarifies the role of Human Ecology, and similar disciplines, by bringing ethical and justice considerations into the assessment of different interventions to promote sustainability. Blending natural, social and cognitive sciences with dynamical systems theory, the authors offer systems approaches that are accessible to all, from the undergraduate student in environmental studies to policy-makers and practitioners across government, business and community.