Public international law regulates primarily inter-state relationships. It is the public international law which determines the rights and obligations of states in relation to each other. In addition to states, also international organisations, and to a certain extent even individuals, are subject to the rules of public international law, for example, as regards human rights.

The fundamental principle of public international law is that states are sovereign entities which should be treated alike from the legal point of view. In contrast to national law, public international law constitutes a horizontally oriented legal system, in which legal rules are created and implemented by the states themselves. This is so because the states give their consent to be bound by a certain rule. Consent can be given in writing (by means of an agreement, treaty, convention, pact, official record) or by means of judicial case law (customary law). No state can be forced, in principle, to enter into an agreement against its will, but states are usually bound by rules which are generally considered as part of customary law.

Public international law examines questions such as, for example: how are legal rules created as well as when and why do states consider themselves to be bound by these rules? When can a new entity be recognised as a new state? What is the relationship between national and international law? In what way may a state exercise its power in its own territory? Who enjoys immunity from the exercise of public authority by the state? When can a state be held responsible for an act or omission? How can the international community control another state’s treatment of its own citizens? When and how can military power be used in international conflicts? What are the rights and obligations of a state as regards international waters or interplanetary space? Are there any global rules for the protection of the environment? How can individuals guilty of crimes against humanity be brought to justice?