International Law as a source of Constitutional Law

This article has the purpose to prove that the Customary International Law and the Conventional International Law are sources of Constitutional Law. First, it analyses the matter of the relations between International Law and National or Domestic law according with the theories dualism and monist and international decisions. Then, it studies the reception and the hierarchy of International Customary and Conventional Law to Domestic Law including Constitution. This matter has been studied according with several Constitutions and the international doctrine. Then, it considers the constitutional regulations about international law in the Constitution of the Republic of Colombia. The general conclusion is that International Law is incorporated in domestic law according with the Constitution of each country. But every state has the duty to carry out in good faith its obligations arising from treaties and other sources of International Law, and it may not invoke provisions in its Constitutions or its Laws as an excuse for failure to perform this duty. Accordingly, state practice and decided cases have established this provision, and the same rule is established in articles 27 and 46 of the Vienna Convention on Law of Treaties of 1969.