Book excerpt: The United Nations in the 21st Century, Sixth Edition, provides a comprehensive yet accessible introduction to the UN. It explores the historical, institutional, and theoretical foundations of the UN as well as major global trends and challenges facing the organization today, including changing major power dynamics, new threats to peace and security, the migration and refugee crises, the COVID-19 pandemic, and the existential challenges of climate change and sustainability. Thoroughly revised and expanded, it contains two new chapters on the UN and the environment and on human security, including issues of health, food security, global migration, and human trafficking. There is enhanced analysis of theoretical perspectives on post-colonialism, feminist theory, constructivism, and non-Western views. New content has also been added on the UN's budget crisis, public-private partnerships, and the role of women in the organization. By examining the UN as an intergovernmental organization facing the broader need for global cooperation to address economic, social, and environmental interdependencies alongside the threats posed by rising nationalism and populism, this popular text is the perfect reference for all students and practitioners of international organizations, global governance, and international relations.