The secondary sources will cite relevant sources of international law. International law materials can be found on websites of intergovernmental organizations, through electronic databases such as HeinOnline, or in print format. 

The International Court of Justice sets out the sources of international law in Article 38 of the Statute of the ICJ in a hierarchical format as follows: 

• international conventions, whether general or particular, establishing rules expressly recognized by the contesting states;
• international custom, as evidence of a general practice accepted as law;
• the general principles of law recognized by civilized nations;
• subject to the provisions of Article 59, judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law.

Treaties, custom, and principles of law are sometimes referred to by lawyers and librarians with a common law background as "primary sources" of international law. Judicial decisions and the teachings of publicists are sometimes referred to as "secondary sources" or evidence of international law rules.