Marine protected areas (MPAs), referred to areas where human activities are restricted to varying degrees with the primary objective of ocean conservation, have emerged and been widely used as one of the most politically and scientifically acceptable approaches to marine protection over the last few decades. Nevertheless, there have been statistics suggesting that MPA “paper parks”, which are merely legislated on paper but offer little actual protection in waters, make up a significant portion of the global tally. This doctoral thesis therefore asks whether international law provides legal protection for MPAs as a conservation technique and how to strengthen MPA legislation. By employing a finer-grained methodology, alongside an instrument-by-instrument approach as well as the research methodology of case studies and law cases, the outcome of the dissertation consists in mapping the variety of existing MPA forms, then providing an overview of different types of MPAs extracted from a vast population of some 17,000 examples. This survey seeks to identify “misuses” or non-conservation uses pursued through the declaration or operation of an MPA. A sectorial perspective, i.e. an analysis of legal interactions focusing on synergies and conflicts between MPAs and selected economic activities at sea namely fishing, shipping, submarine cables, and offshore extractive activities, is adopted in this dissertation. As the sectorial stressors of MPAs, these activities must be successfully managed for an MPA to be effective from a conservation standpoint. The sectorial analyses aim to lay the groundwork for the integrated approach of multiple-use ocean zoning and marine spatial planning for the effective management of MPAs. The dissertation asserts that treaties prove to play a role in defining the number of legal regimes applicable to MPAs. It is also argued by reference to case studies and examples that there is a sort of “general” or “common” international law of MPAs, and when such general law (the principles, rules, and standards of international law) is contravened (e.g. unilaterally defined MPAs for dubious purposes) there is an outcry. Similarly, when too much room is left to non-conservation activities, it also raises inconsistencies. The need for an integrated approach is therefore justified. By contrast, multilateral and genuine conservation MPAs are consistent with the wider foundations of general international law. Thus the integrated approach to MPAs which builds upon international law can indeed be conductive to avoiding MPA misuses, and simultaneously, to supporting the conservation purposes pursued by the declaration of MPAs.