This thesis examines the post-crisis regulation of the UK private security industry. It seeks to explain the puzzling shift from hierarchical command-and-control to more hybrid patterns of private security regulation in the post-crisis era (2008-2018). What is puzzling about this shift is that these more complex, networked patterns of private security regulation have emerged in the absence of any formal regulatory reform. To analyse and explain these 'hidden' changes in private security regulation - and departing from existing approaches - this thesis constructs a novel theoretical framework from the literature on private security regulation, political institutionalism and bureaucratic politics. Through this lens - and drawing on original interview and documentary analysis - this thesis explains the 'hybridisation' of private security regulation with reference to the Security Industry Authority negotiating external pressures stemming from the shifting post-crisis environment in order to achieve its objectives and maintain its autonomy. It argues that in the context of a fixed legal mandate and increasing legal, resource and regulatory constraints, the SIA has sought to enhance its capacity to achieve its regulatory objectives by supplementing its (limited) formal authority with the informal authority of non-state actors and networks. The implication is that in the post-crisis era, private security regulation has been gradually redirected from its original purpose of protecting the public from private security towards the goal of protecting the public with private security. Through its examination of the 'hybridisation' of private security regulation in the post-crisis era, this thesis contributes not only to a deeper understanding of the political and dynamic nature of private security regulation but also to key contemporary academic debates on the state, policing and regulation.