Chapter 9 examines security exceptions known to many investment treaties. These clauses are designed to expand the host state's room to adopt certain measures in the pursuit of security interests without having to compensate foreign investors. While seeking to enhance legal security, security exceptions remain ambiguous and controversial in many respects. The chapter conducts a comprehensive study of relevant case law, not only of recent investor-state arbitrations but also of the World Trade Organization’s dispute settlement body. On this basis, it offers guidance on the interpretation of elements common to security exceptions as well as on the level of review to be conducted by investment tribunals. The chapter delineates the limits of state discretion and proposes that international humanitarian law should have an impact on the application of security exceptions in the context of military operations. In evaluating drafting options, the chapter further assesses the merits of including a security exception in investment treaties and makes suggestions as to their wording and function.