Worker organizing has long been a cornerstone of efforts to secure fair pay, decent working conditions, and protection against the insecurities of market economies. By acting collectively, workers have historically been able to counterbalance employer power and win improvements to work and life. Yet such gains have always been partial and contested, and the settlements between labour and capital remain under constant pressure. In the wake of rapid social, political, technological, and environmental change which is reshaping work and challenging the institutions and practices that have underpinned worker collective action, this edited volume explores the evolving landscape and practices of worker organizing in the twenty-first century. In the context of developments that have exposed the limits of existing models of worker representation and protection - from the erosion of union density in many countries, to the rise of precarious and platform-based work, to climate crisis and pandemic fallout - the chapters in this volume examine the innovative strategies, organizational forms, and solidarities that workers are forging in response to intensified inequalities and new modes of exploitation. The contributions draw on diverse theoretical perspectives and examine a range of empirical settings to shed light on the forms, dynamics, and outcomes of worker collective action in contemporary contexts. Recentring worker organizing as a vital force in shaping a more equitable, sustainable, and democratic world of work, Worker Organizing is a timely and compelling volume for scholars of the sociology of work and labour, organizational studies, social movements, and beyond.