Waisted outlines the fascinating and misunderstood biology of fat (i.e., adipose tissue). This controversial, much-maligned organ plays a crucial yet curiously overlooked role in the global obesity crisis currently wreaking havoc on the world’s healthcare systems and economies. Attaining a better appreciation of the biology of fat, its social meanings, and how these intersect is essential for improving the world’s physical and mental health. Far from being a passive layer of blubber under the skin, fat plays a highly dynamic role in energy metabolism, reproductive health, and immunity, with these links having ancient origins in the evolution of modern humanity. Waisted provides a comprehensive, evidence-based perspective on the biology of fat and its crucial role in human evolution, health, disease, and society. Waisted draws upon biomedical, epidemiological, and evolutionary research to understand adipose tissue biology and the striking relationship between body fat distribution and health outcomes. Waisted demonstrates the practical implications of key conceptual points through relatable real-world cases and highlights how seemingly disparate common and rare diseases may be underpinned by adipose tissue dysfunction. Overall, Waisted covers a wide breadth of material that challenges and reframes the generally negative perspective of fat to highlight the underappreciated importance of adipose tissue biology in humans.