This paper contributes to Contradictions’ “Conceptual Dictionary” by exploring the history, theory, and practical implications of the notion of “ecosocialism”. The first part of the text follows the historical and ideological development of ecosocialism and examines its broad and diverse anarchist, feminist, and Marxist roots. This part of the text also introduces key authors of ecosocialism and their work. The subsequent part identifies critical ideas and offers a theoretical overview of scholarly and political debates about the relationship between ecosocialism and other environmental and socio-critical approaches. Finally, the text further explores the practical and political vision of the concept and reflects on the future of ecosocialist strategy.