Customary law and conventional law are primary sources of international law.

Customary international law results when states follow certain practices generally and consistently out of a sense of legal obligation. Recently the customary law was codified in the Vienna Convention on the Law of Treaties. Conventional international law derives from international agreements and may take any form that the contracting parties agree upon. Agreements may be made in respect to any matter except to the extent that the agreement conflicts with the rules of international law incorporating basic standards of international conduct or the obligations of a member state under the Charter of the United Nations. International agreements create law for the parties to the agreement. They may also lead to the creation of customary international law when they are intended for adherence generally and are in fact widely accepted. Customary law and law made by international agreement have equal authority as international law. Parties may assign higher priority to one of the sources by agreement. However, some rules of international law are recognized by international community as peremptory, permitting no derogation. Such rules can be changed or modified only by a subsequent peremptory norm of international law.

General principles common to systems of national law is a secondary source of international law. There are situations where neither conventional nor customary international law can be applicable. In this case a general principle may be invoked as a rule of international law because it is a general principle common to the major legal systems of the world and not inappropriate for international claims.

International law establishes the framework and the criteria for identifying states as the principal actors in the international legal system. As the existence of a state presupposes control and jurisdiction over territory, international law deals with the acquisition of territory, state immunity and the legal responsibility of states in their conduct with each other. The law is similarly concerned with the treatment of individuals within state boundaries. There is thus a comprehensive regime dealing with group rights, the treatment of aliens, the rights of refugees, international crimes, nationality problems and human rights generally. It further includes the important functions of the maintenance of international peace and security, arms control, the peaceful settlement of disputes and the regulation of the use of force in international relations. Even when the law is not able to stop the outbreak of war, it has developed principles to govern the conduct of hostilities and the treatment of prisoners. International law is also used to govern issues relating to the global environment, the global commons such as international waters and outer space, global communications and world trade.

Whilst municipal law is hierarchical or vertical, with the legislature enacting binding legislation, international law is horizontal, with all states being sovereign and theoretically equal. Because of this, the value and authority of international law is dependent upon the voluntary participation of states in its formulation, observance, and enforcement. Although there may be exceptions, most states enter into legal commitments to other states out of enlightened self-interest rather than adherence to a body of law that is higher than their own.