This dissertation employs the doctrinal method of research to appraise the doctrine of Self defence as one of the fundamental principles of International law, and as one of the exceptions to the prohibition on the use of force. To this end, this dissertation centers on Article 51 of the United Nations Charter which provides for the right of self defence in International law. The dissertation contends that the provisions of Article 51 have generated some controversies among scholars of International law. These controversies have tended to obscure the scope of self defence in International law. The major problem of this research is that it is not clear whether Article 51 has abrogated or preserved the doctrine of anticipatory Self defence in Customary International law. This problem has been complicated by the use of the phrases ‘inherent right of individual or collective self defence’ and ‘armed attack’ in Article 51. The question therefore is that ‘does international law expect a State to do nothing where it is a target of an imminent attack’? The objective of this dissertation therefore is to examine the relationship between Article 51 and rules of customary International Law, and the circumstances in which the right of self defence can be exercised. The dissertation makes some findings by submitting that the doctrine of preemptive Self defence is contrary to Articles 2(4) and 51 of the Charter which prohibits unilateral use of force. Furthermore, both Article 51 and customary international law provide different rules for the exercise of the right of self defence .The writer suggests that there is urgent need for an amendment of Article 51 to bring it in line with current global challenges to global security. The phrase ‘armed attack’ should be well defined and the concept of collective self defence should be deleted from Article 51.