A treaty under international law is an agreement entered into by sovereign states and international organizations.

Treaties are comparable to contracts, in the sense that both are means of willing parties assuming obligations among themselves.  In both treaties and contracts, a party to either that fails to live up to their obligations can be held liable under international law.  The central principle of treaty law is expressed in the maxim pacta sunt servanda—”pacts must be respected”.

Treaties under international law may either be bilateral or unilateral.  Multilateral treaties establish the rights and obligations between the parties.  Moreover, multilateral treaties are often, but not always regional.

Bilateral treaties by contrast are negotiated between a limited number of states, most commonly only two, establishing legal rights and obligations between those two states only.

Though on entering into a treaty, the states become generally bound by the terms of the treaty, it is possible to modify, alter or amend treaty obligations.  These are possible through Amendments, Protocols and Reservations.  Reservations are unilateral statement purporting to exclude or to modify the legal obligation and its effects on the reserving state.  These must be included at the time of signing or ratification.  In other words, a party cannot add a reservation after it has already joined a treaty.

An existing treaty can be amended in three ways.  First, formal amendment requires States parties to the treaty to go through the ratification process all over again.  The re-negotiation of treaty provisions can be long and protracted, and often some parties to the original treaty will not become parties to the amended treaty.  When determining the legal obligations of states, one party to the original treaty and one a party to the amended treaty, the states will only be bound by the terms they both agreed upon.  Treaties can also be amended informally by the treaty executive council when the changes are only procedural, technical, or administrative.  Finally, a change in customary international law can also amend a treaty, where state behavior evinces a new interpretation of the legal obligations under the treaty.

In international law and international relations, a protocol is generally a treaty or international agreement that supplements a previous treaty or international agreement.  A protocol can amend the previous treaty, or add additional provisions.

Interpretation of a treaty is also a subject of importance under international law.  The language of treaties, like that of any law or contract, must be interpreted when the wording does not seem clear or it is not immediately apparent how it should be applied in a perhaps unforeseen circumstance.  The Vienna Convention states that treaties are to be interpreted “in good faith” according to the “ordinary meaning given to the terms of the treaty in their context and in the light of its object and purpose.”

No one party to a treaty can impose its particular interpretation of the treaty upon the other parties.  Consent may be implied, however, if the other parties fail to explicitly disavow that initially unilateral interpretation, particularly if that state has acted upon its view of the treaty without complaint.  Consent by all parties to the treaty to a particular interpretation has the legal effect of adding an additional clause to the treaty – this is commonly called an ‘authentic interpretation’.

A party to a treaty may also put an end to it either by withdrawal, suspension or termination.  While many treaties expressly forbid withdrawal, other treaties are silent on the issue, and so if a state attempts withdrawal through its own unilateral denunciation of the treaty, a determination must be made regarding whether permitting withdrawal is contrary to the original intent of the parties or to the nature of the treaty.

If a party has materially violated or breached its treaty obligations, the other parties may invoke this breach as grounds for temporarily suspending their obligations to that party under the treaty. A material breach may also be invoked as grounds for permanently terminating the treaty itself.