This chapter will consider a range of concepts in self-defence and their application at sea. It will look first at the principles enshrined in article 51 and also the Caroline Principles. It will then look at the cases that have applied these principles at sea – the Corfu Channel Case, the Nicaragua Case, the Saiga Case, the Oil Platforms Case and the South China Sea Arbitration. This chapter will argue that it is possible to derive some clear rules from these cases, even if they do not deal comprehensively or precisely with all of the relevant issues. Large areas of uncertainty remain about the lawful use of force in incidents at sea involving warships, and these areas will always remain so because of the inherent tensions in the concept of self-defence.