Transnational private regulation constitutes a key element in contemporary debates about regulatory governance. This PhD study examines the interplay between public and private enforcement mechanisms in ensuring compliance with transnational private standards regulating business activities between and across jurisdictions. It does so by conducting a comparative case study analysis in the issue areas of advertising and food safety. It describes the organisation and operation of private mechanisms employed to enforce transnational private regulation in these areas, assesses the interplay between these private mechanisms and public enforcement, and identifies the institutional factors that enable this interplay. On the basis of the findings of these two case studies, the study argues that public and private enforcement mechanisms of transnational private regulation operate principally as complementary means. The concept of complementarity denotes the combination of different mechanisms and activities with the purpose of enhancing regulatory outcomes. This can be achieved either by reinforcing the operation and effects of other mechanisms and activities or by compensating for their deficiencies. As the case studies demonstrate, public enforcement action, or the latent threat thereof, may effectively bolster the capacity of private mechanisms to attain rule compliance. At the same time, however, effective private enforcement mechanisms enable public enforcers to economize on the deployment of scarce enforcement resources. Several preconditions and institutional settings apply to enable forms of public-private complementarity. First, there should be a substantive overlap between the objectives and standards of public and private regulation. Second, the institutional design of public enforcement should enable public authorities exercising administrative enforcement powers to coordinate their activities with private mechanisms. Third, private enforcement should meet minimum standards of due process. Finally, public and private enforcers should design proper mechanisms of data sharing and information management to enable the adequate coordination of responses to non-compliance.