In seeking to fulfill its mission, the International Maritime Organization (IMO) is dependent on the capabilities of its Member States. While flag States remain primarily responsible for ensuring compliance of their registered vessels with instruments adopted under that United Nations specialized agency's aegis, coastal States also play a competing but complementary role in the balance of jurisdiction over ships. In particular, the right to regulate the vessel's intent on visiting the port, or its presence therein, is often relied upon to account for the limitations of flag State enforcement. The capacity to act as a port State under international law is the basis for certain implementation mechanisms stemming from treaty provisions on port state control, as well as for innovative practices of port States, either collectively or individually. This chapter provides the reader with an overview of the role of port State jurisdiction in IMO law and practice, and the challenges that can arise in the complex relationship between port States and the IMO.