Everyone has experienced loneliness – perhaps briefly – perhaps for many years. This handbook explores why people of all ages can become lonely, and features steps that can be taken by individuals, communities, and entire societies to prevent and alleviate loneliness. Chapters present rigorous scientific research drawn from psychology, relationship science, neuroscience, physiology, sociology, public health, and gerontology to demystify the phenomenon of loneliness and its consequences. The volume investigates the significant risks that loneliness poses to health and the harmful physiological processes it can set in motion. It also details numerous therapeutic approaches to help people overcome loneliness from multiple perspectives, including traditional and cognitive psychotherapy, efforts to connect individuals to their communities, and designing communities and public policies to create a greater sense of social connection. Using accessible terminology understandable to a non-medical audience, it is an important work for social science scholars, students, policymakers, and practitioners.