The book explores how diversity in United Nations’ peace mission composition affects peacekeeping effectiveness. It identifies four key dimensions of composition: Blue Helmets’ field diversity, top mission leadership diversity (between Force Commander and Special Representative of the Secretary General), vertical leadership distance (Leadership-Blue Helmets), and horizontal distance with the local population. Each dimension of diversity of mission is measured as linguistic, geographical, and religious distance. Our book conceptualizes original mechanisms—i. resolve commitment; ii. informative trust; iii. informative communicability; iv. skilled persuasion—through which diversity can shape mission effectiveness such as trust, communicability, deterrence, and persuasion. It then evaluates each dimension separately through three pathway case studies—the UN missions in Lebanon, in Mali, and in the Central African Republic—and quantitative analyses based on a global dataset of peacekeeping operations deployed since the end of the Cold War. The book finds that diversity of Blue Helmets and diversity of top leadership may increase the mission’s capacity to reduce battle-field violence and civilian victimization. At the same time, the effects of diversity are contextual and contingent. In fact, looking at the relation between peacekeepers and Force Commanders, proximity between them is generally associated with better performances. Furthermore, homogeneity between local populations and peacekeepers, or low distance between them, is also related to low levels of hostility and casualties. This book crucially demonstrates why diversity of mission composition is a key variable to consider when trying to enhance peacekeeping effectiveness.