This chapter examines NATO’s evolving engagement with environmental security, tracing its transformation from a traditional military alliance to an organization increasingly addressing climate-related threats. Beginning with the 1991 Strategic Concept’s recognition of environmental challenges, the study analyzes NATO’s progressive institutionalization of environmental security through key policy milestones, including the 2010 Strategic Concept, the 2021 Climate Change and Security Action Plan (CCSAP), and the 2022 Strategic Concept that identifies climate change as “the defining challenge of our time.” The chapter undertakes a comprehensive literature survey examining scholarly debates on whether NATO’s climate initiatives represent transformative change or remain primarily reactive. The analysis explores three dimensions: theoretical approaches (securitization, climatization, and risk governance), operational implications (infrastructure vulnerability, emissions, disaster relief), and regional case studies. Despite significant progress in recognizing climate as a ‘threat multiplier,’ persistent challenges remain, including uneven implementation across member states, tensions between military imperatives and sustainability goals, and gaps in emissions reporting. The chapter concludes that NATO’s success depends on standardizing environmental protocols, strengthening civil-military collaboration, and mainstreaming climate considerations into strategic planning, thereby balancing collective defense with environmental stewardship in the twenty-first century.