Under many international agreements, states have negotiated language which provides that even when states have entered into treaty commitments, such commitments do not prevent them from taking measures in order to protect their essential security interests. How often are provisions on essential security interests found in investment agreements? What is their scope? Is the state entitled to be the sole judge for invoking these provisions, i.e. are they self-judging? Is there relevant customary international law on this issue? How have arbitral tribunals interpreted essential security provisions? The present article focuses on these questions. It analyses: i) the frequency and scope of these provisions in international investment agreements and instruments to which OECD members are party; ii) the way customary international law bears on this issue; and iii) the views of arbitral tribunals who expressed themselves on these issues in specific cases.