This article develops a research framework for understanding workers and their unions as strategic actors in the climate transition, emphasising the centrality of their agency. While climate change poses profound challenges such as job losses, work reorganisation, and health risks, workers are often described as passive recipients of state or employer-led decisions. We conceptualise union engagement along two dimensions: the strategies they pursue (‘what’) and the factors shaping these strategies (‘why’). Drawing on existing literature and empirical observations, we first identify five generic strategies: abstentionist, unconditionally supportive, conditionally supportive, oppositional, and transformative, mapped along two dimensions: agreement with climate policies and engagement in climate action. Then, we propose a framework integrating internal factors, including worker interests, union power resources, and strategic capabilities, with external influences such as structural power, institutional power, societal discourse, and employer strategies.