This thesis is about multi-stakeholder initiatives that seek to regulate the human rights impacts of global apparel supply chains (Apparel MSIs). MSIs have the aim of improving human rights for millions of apparel workers worldwide, but after two decades they show little evidence of such improvement. Civil society critics argue that MSIs are ineffective, unreformable private regulation that is not fit-for-purpose and lacks legitimacy. This thesis argues that Apparel MSIs still perform a valuable regulatory function, however they must adopt new regulatory approaches. These include moving beyond social audit as a regulatory technique, expanding stakeholder participation and better measuring and communicating impact. MSIs must transform to realise their aim of improving apparel workers human rights and consequently preserve their legitimacy. To understand and contribute to this transformation the thesis method incorporates existing literature; it applies theoretical frameworks; and the insights of original empirical research. From the latter the voices of worker advocates, union leaders and academics reveal recent and promising regulatory innovations and changes in MSIs. Along with this empirical research, the original contributions of this thesis are to emphasise the interconnected nature of legitimacy criteria and assess the overall legitimacy of Apparel MSIs in the light of the functional model adopted by each MSI. This legitimacy analysis is supported by the regulatory theory of responsive regulation, which explicitly contemplates self-regulatory forms like MSIs. The original contribution of bringing responsive regulation to bear on Apparel MSIs, provides new insights into how they can bolster their regulatory effectiveness and legitimacy. Interviews undertaken with key stakeholders provide a sociological perspective to this analysis. Interview data also drive the final recommendations for reform which coming from MSI stakeholders point to recent innovations in private regulation as a more promising alternative. Given the opportunity to build a more just world after the Covid-19 pandemic, these recommendations could not come at a more critical juncture.