In this chapter, Róisín Burke provides a comprehensive overview of the interest of armed forces deployed on peace operations or other missions to ensure effective investigation and prosecution of serious international crimes committed by their members in host States. She draws lessons from past incidents and identifies a range of reasons for accountability: ethical and moral values, self-regulation and internal discipline, the image of the armed forces and their States, their relationship with host State populations and with their home public, retention of control by military justice systems, operational effectiveness and legitimacy, and the promotion of the rule of law.