This paper sheds light on the ‘private-vs.-public enforcement’ debate. In the first part, using EU internal market law as an example, it will be illustrated that the (relative) effectiveness of private and public instruments of enforcement is significant not only from a policy point of view, but in fact also for the application of legal principles. An analysis of the ECJ’s case law reveals that there is no general obligation on the part of the Member States to provide in each context for mechanisms of both private and public enforcement to guarantee a sufficient level of compliance with substantive EU law. In the second part, taking a functional perspective, an overview of the ‘private-vs.-public enforcement’ debate is provided. An essential finding is that neither a model of purely private enforcement nor a model purely public enforcement can be constructed as a first-best-enforcement solution. The most one can hope for is a robust second-best enforcement strategy that will often, but not necessarily, require a mix of elements of private and public enforcement.