Human rights due diligence (HRDD) has emerged as a dominant frame through which to conceptualise and operationalise responsible business conduct with respect to workers’ rights in transnational supply chains. Legislation mandating HRDD is now found in a number of European countries and is on national regulatory agendas in other regions of the world. Many scholars, practitioners, and activists are welcoming—and, indeed, actively calling for—further legalisation of the concept, believing that this will broaden and deepen respect for human rights. This book critiques HRDD as an emerging instrument of transnational labour law. Proceeding from the observation that the concept as originally articulated in the UN Guiding Principles on Business and Human Rights is open to multiple interpretations, the book examines contests that have taken place globally over the role and status of the concept. It also considers the implications of HRDD’s ascension for transnational labour law as a distinct field of law, scholarship, and activism. Combining insights from transnational governance and business regulation with empirical analysis, this book reveals that HRDD is not being institutionalised at either the global or national level in a way that renders it a transformative or even robust mechanism of transnational labour law. It also draws attention to the important ways in which the rise of the concept is leading to subtle shifts in the configuration of actors and institutions in transnational labour governance.