Treaties and international agreements are agreements between two or more countries or international organizations that are governed under the principles of international law. Treaties can cover a variety of topics related to international relations, including trade, defense, the environment, and human rights. Treaties have been an integral part of international law and foreign policy for centuries. One of the oldest treaties still observed today is the Treaty of Windsor External, between the United Kingdom and Portugal, which came into force on May 9, 1386. This treaty contains provisions for mutual defense, commercial activities, and even immigration. Treaties today have evolved as society has advanced. One of the most famous treaties the United States entered into, the Treaty with Tripoli (1796) dealt with piracy in the Mediterranean Sea. In a more recent example, the United States has become a party to a treaty on cybercrime, the Council of Europe Convention on Cybercrime.

Treaties are typically divided into two main categories, multilateral and bilateral. A multilateral treaty is a treaty involving more than two parties, while a bilateral treaty involves an agreement between two parties. There is often a resource overlap when researching multilateral and bilateral treaties, however researchers can find distinct resources focusing on each category. Treaties are entered into based on domestic legal procedures. In the United States, for example, a treaty is considered and consented to by the U.S. Senate, which authorizes the President to ratify the treaty.

Once you determine if you are working with a bilateral or multilateral treaty, you can then identify the primary treaty document using a citation, or by searching a depository, database, or compilation by category, subject, or keyword. When researching treaties, you may encounter terms that have specialized meanings within this area of research. For example "ratification" of a treaty occurs when the parties consent to be bound by the agreement. An "accession" to a treaty occurs when a party consents to be bound by a previously ratified agreement. You may also see the term "reservation" while researching; a reservation is an act by a party where they exclude a certain provision of a treaty, but agree to the rest of the provisions. Awareness of these terms and their meanings will enable you to identify treaties and related instruments and acts more effectively.