The focus of this chapter is the criminal use of submarines for smuggling, terrorism and other criminal offences. The chapter begins with an explanation of international law principles relevant for policing in varied maritime zones and over different persons and acts. Submarine involvement in maritime law enforcement is not common and the discussion recognises how submarines may be limited to information gathering and the commencement of hot pursuit. Instead, the deployment of submarines for smuggling has prompted closer consideration of laws addressing transnational crime and underlines the important role of domestic law. US legislation dealing with drug trafficking by submersibles is assessed in this regard. Potential terrorist use of submarines, extending to possible mutinies, are regulated through different international instruments. We highlight the need for States to participate in these treaty regimes, as well as ensuring their national laws are properly in place to deal with this increasing criminal activity.