Whether as the primary goal or a key operational objective, protecting civilians from systematic violence and mass atrocity creates unique considerations for military operations. This document is the culmination of several years of research by the Stimson Center’s Future of Peace Operations program on those considerations, and what such operations require to succeed. It is informed by research into the fundamental drivers and dynamics of violence against civilians; analysis of military operations that attempted to protect civilians across a variety of regional, institutional, and theatre contexts; and wide consultations with experts, policy makers, and practitioners who have worked on these operations. In particular, it draws on the results of a workshop organized by the Stimson Center and held at the UK Defence Academy in September 2009. That event brought together scholars, doctrine writers, and military officers from around the world for a series of planning exercises around scenarios involving threats to civilians of varying nature and intensity. The result was a wide-ranging discussion of the challenges and key concepts for military operations tasked with the protection of civilians. The lessons identified in the discussions were combined with additional research and consultation to produce the Principles captured here.