This paper aims to peruse the two predominant principles of International Law: Jus Cogens & Obligations Erga Omnes. It will provide a synoptic outline of these two principles by whirling on their relationship and dissensions. It will discuss their recognition in the 1986 Vienna Convention on the Law of Treaties, Articles on Responsibility and Barcelona Traction Case. It will accentuate their effects, historical origins and the reciprocal fields of application.