This book, edited by Professor Helena Torroja, grew out of her ongoing commitment to the mandate of the Working Group on the use of mercenaries (OHCHR), with which she has actively collaborated since shortly after its creation by the Human Rights Commission in 2005 (Res. 2005/2). The Working Group has benefited from her expertise with various activities carried out in fulfillment of our mandate, according to which we are to monitor mercenaries in all their manifestations and study mercenary-related trends and activities in different parts of the world and their impact on human rights, particularly on the right of peoples to self-determination. The collaboration has primarily focused on the part of the mandate concerning the impact of the activities of private military and security companies (PMSCs) on these rights and the regulatory frameworks governing such companies around the world, including their gaps, shortcomings, and good practices. The present book was undertaken at the initiative of Professor Torroja in response to the growing concern of the international community over the threat posed by the criminal activities of mercenaries to peace and security in developing countries, particularly in conflict zones. It likewise aims to address concerns over the role played by PMSCs, which have diversified their activities from the provision of military assistance, consultancy, and security services to other areas in which they also affect the enjoyment of human rights and are rarely held accountable for human rights violations. The work of the experts invited to participate, all of whom have extensive experience in the study of these phenomena, reflects the urgent need to make effective progress on the strengthening of the international legal framework for both phenomena at a time when, as recognized by the Human Rights Council in its Resolution on the Working Group, “armed conflicts, terrorism, arms trafficking and covert operations by third Powers encourage, inter alia, the demand for mercenaries and for private military and security companies on the global market” (Res. 33/4 de 2016).