This chapter provides first a discussion of how maritime security has been conceptualized and theorized and how the field has evolved. It discusses the more particular debates on dedicated maritime security issues: piracy, terrorism, smuggling, environmental crimes and the protection of critical maritime infrastructure. Although the oceans have featured occasionally in the literature on security, peace and development, it is fair to say that for decades scholars were suffering from what some have referred to as collective ‘seablindness’. A range of maritime insecurities have been extensively analysed. These include piracy; terrorism; various forms of smuggling; environmental crimes, hereunder illegal fishing; as well as a nascent literature on maritime critical infrastructures. With ongoing crises in different parts of the world’s oceans, maritime insecurity will continue to be recognized as one of the core dimensions of violence and insecurity. Maritime security also needs to be seen in the context of other international policy areas.