What is treaty law?

Treaty law, along with customary international law, are considered two of the main sources of international law.   

The Vienna Convention on the Law of Treaties (VCLT) (1969), is the main legal instrument related to treaty law and sets out the fundamental rules relating to treaties (also referred to as conventions) and their interpretation.    

According to the VCLT, a “treaty” means an international agreement concluded between States in written form and governed by international law. International organisations may also be members of a treaty. Treaties are legally binding to the parties thereto, which willingly ratify them and accept the obligations they prescribe. The binding nature of treaties ‘pacta sunt servanda’ (‘agreements must be kept’) is a foundational  rule of customary international law (see below).   

Ukraine follows a monist approach to international law, meaning that it views international law as part of its domestic legal order. As such, it is highly receptive to international law. Article 9 of the Constitution of Ukraine reads that “International treaties that are in force, agreed to be binding by the Verkhovna Rada of Ukraine, are part of the national legislation of Ukraine”. This means that international treaties accepted or ratified by the Verkhovna Rada of Ukraine, become part of Ukraine’s national legislation without having to be transposed into specific domestic laws. Additionally, ratified treaties have higher hierarchical status than national law, as stipulated also in Article 19 of the Law on international agreements of Ukraine. The Constitutional Court of Ukraine has confirmed the monist approach through the principle of friendly attitude towards international law. It has stated that it takes into account the ratified treaties by the Verkhovna Rada of Ukraine, as well as the practice of interpretation and application of these treaties by the international bodies whose jurisdiction Ukraine has recognised [Judicial Benchbook, paras. 170-171, 174].