Human rights due diligence (HRDD) is set to become a mandatory obligation imposed on many larger businesses by a variety of governments globally. This briefing reports on the first detailed empirical research into HRDD, based on interviews with leading practitioners in the field. 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; 3) the nature of the competition which takes place between HRDD practitioners. It therefore provides recommendations for how mandatory HRDD (mHRDD) laws can empower ey actors to effectively hold companies accountable for the HRDD they produce. If these laws are ineffective, it argues for the setting up of an independent body to have lead responsibility for overseeing the HRDD process internationally.