This book examines both primary and secondary rules of international law. It identifies the pertinent primary obligations on three categories of states – the hiring state, the home state and the host state of a PMSC – and assesses how particular PMSC misconduct may give rise to the international responsibility of these states for a violation of their obligations. This chapter lays the general theoretical groundwork for the book by outlining the basic normative structure of the international legal system and critically examining how the law of state responsibility operates within that systemic context.