The understanding on how international law is regulated is essential to be understood. There are many regulations, especially in international sphere that regulate states. This book captures the most fundamental rules on international law that needs to be understood by anyone who wishes to study about international law. The book is divided in three chapters. The first chapter discuss The Vienna Convention on the Law of Treaties, also known as the VCLT. In the second chapter, the author describes about The Convention on the Prevention and Punishment of Crimes against Internationally Protected Persons, including Diplomatic Agents. Meanwhile, in the third chapter, the author discusses The Vienna Convention on Succession of States in Respect of Treaties. In each chapter, there are four parts that assist the understanding of those regulations which are the introduction, substance of the regulation, the importance of the regulation, legal terms identification, and the text of the regulation itself.