There is a growing recognition of the role played by loneliness in poor health and mortality. There has been significant attention paid to loneliness in later life, particularly with regard to its effect on health. However, there is relatively less research on how loneliness affects health in childhood, adolescence, and early adulthood. This is particularly important, as the health impact of loneliness early on in life may persist into later life. The present chapter aims to summarize evidence on loneliness and a range of health and cognitive outcomes using a developmental perspective. The chapter also looks at behavioral mechanisms such as smoking, alcohol consumption, physical activity, and sleep, through which loneliness may affect health. Lastly, the chapter outlines some recommendations for future research.