This chapter discusses maritime security, reviewing relevant law of the sea concepts. The modern law of the sea encompasses both functional and zonal approaches: the question of the law applicable to any situation thus involves analysis of both the activity in question and where it is conducted. Several traditional law of the sea enforcement techniques are also being adapted to new challenges. Principal amongst these is the doctrine of port State jurisdiction. The chapter then surveys a number of challenges in the maritime domain. It looks at three major themes cutting across these various silos. The first is Maritime Domain Awareness; if the law of the sea regulates who may do what and where, then a challenge for enforcement is knowing who is doing what and where. The second theme is the turn to informality. Most new maritime security initiatives do not involve creating new organizations or legal instruments; responses to collective or regional challenges tend now to occur through informal coalitions. Finally, the broadening of maritime security brings a wider range of non-State or ‘grey zone’ actors into the picture, including migrants, seafarers, transnational criminals, and hybrid private/State actors such as the Chinese maritime militia.