The widespread fragmentation of global supply chains (GSCs) poses considerable challenges for regulating severe forms of labour exploitation, collectively referred to as modern slavery. Recent responses to these challenges include the promulgation of modern slavery reporting (MSR) laws in the UK, California and Australia. In theory, these laws are a promising development geared towards leveraging corporate influence over GSCs to minimise, or where possible, eliminate modern slavery. However, commentators have criticised their design for lack of sanctions and inadequate reporting requirements. Despite this, MSR laws continue to proliferate internationally, underscoring a need for critical inquiry into the antecedents of their design and their practical effectiveness. To that end, this thesis poses the following overarching research question: How do policy-making process dynamics shape the design of modern slavery reporting laws, and how do external stakeholder pressure and actors internal to firms influence their effectiveness? To answer this question the thesis is organised into six chapters. Two introductory chapters define modern slavery and explore the literature on regulatory design. Following this, three empirical studies are presented, each occupying a single chapter. These studies apply policy change theory to explain MSR law design outcomes, stakeholder influence over reporting outcomes, and the how actors internal to the firm shape changes in policy and practice, respectively. The final chapter summarises the central conceptual and practice-related insights of the thesis. Regarding design, MSR law design is shown to be shaped primarily by coalitions of actors through deliberative, associational and ideological strategies throughout the policy change process. Regarding effectiveness, reporting quality is found to depend on the influence of various stakeholder groups, which varies significantly across institutional settings. In addition, policy and practice changes prompted by MSR laws are shown to be contingent on the extent to which organisations empower internal compliance professionals to promote and enact change and how third-party advisers define best practice. Overall, the findings of this study indicate that fulfilment of the aims of MSR laws is far from guaranteed. This will require changes to their regulatory design and more regulator commitment to steering corporate behaviour in the desired direction.