The Montreux Document on Private Military and Security Companies (Montreux Document) was adopted in 2008 by seventeen States to reaffirm and, as far as was necessary, clarify the existing obligations of States and other actors under international law, in particular under international humanitarian law (IHL) and international human rights law (IHRL). It also aimed at identifying good practices and regulatory options to assist States in promoting respect for IHL and IHRL by private military and security companies (PMSCs). Today, fifty-one States and three international organizations have endorsed the Montreux Document. It contains twenty-seven “Statements” – sections recalling the main international legal obligations of States in regard to the operations of PMSCs during armed conflicts. Each statement is the reaffirmation of a general rule of IHL, IHRL or State responsibility formulated in a way that clarifies its applicability to PMSC operations. This article aims to detail the basis of each legal obligation mentioned in the first part of the Montreux Document (Part I). The article follows the structure of Part I, in order to better facilitate its comprehension. The second part of the Montreux Document, relating to good practices, is not covered in this article.