There are two principal theories put forward by scholars on the relationship between international law and municipal law: monism and dualism.

According to monism, international law and municipal law are, in general, the components of one system of law.The main proponents of this theory are Hans Kelsen (1881-1973) and Hersch Lauterpacht (1897-1960). Gerald Fitzmaurice and Joseph Gabriel Starke are other prominent supporters of the approach.

On the other hand, according to dualism, they represent two entirely distinct and self-contained legal systems. The main proponents of this theory are Heinrich Tripel and Dionisio Anzilotti.