The United Nations has been an essential actor in world politics for 75 years. Its entities have eliminated smallpox, protected the ozone layer, promoted arms control, and helped save over 90 million children's lives. Yet, it is frequently criticized as ineffective and antiquated. This book provides a balanced, systematic overview of the UN's contributions and challenges, highlighting its essential roles in global governance as well as areas of redundancy and needed reform. This clear, well-organized reference resource describes the entire UN system-its principal organs, specialized agencies, programs and funds, and key issues of engagement. Entries examine the history of UN engagement, from peace and security to migration and climate change. It moves beyond a simple description of UN entities and assesses the development of ideas (such as sustainable development) as well as responses to changes in world politics, and presents both significant successes and continued challenges.