Human rights due diligence (HRDD) looks set to become a mandatory obligation imposed on many larger businesses by a variety of governments globally. But our understanding of the future potential of HRDD is currently constrained by lack of research into how it is operationalised in practice. This paper fills that gap by providing the first detailed empirical analysis of HRDD on the basis of interviews with practitioners who undertake HRDD, or aspects thereof, for companies. It argues that HRDD has the potential to address both a knowledge problem and an action problem with regard to the human rights performance of transnational corporations (TNCs). But the findings of this research identify three key challenges to making HRDD effective; (1) methodological uncertainty about key aspects of the process (2) power dynamics between critical actors who are charged with undertaking vital aspects of HRDD and (3) the nature of the competition which takes place between HRDD practitioners. Mandatory HRDD laws must empower key actors to effectively hold companies accountable for the HRDD they produce, otherwise more radical regulatory interventions need to be considered.