In January 2018, the UK became the first country in the world to appoint a Minister for Loneliness. This highlights growing concerns about loneliness in societies, and its deleterious effects on individual health and well-being. Loneliness or perceived social isolation is a subjective experience relating to dissatisfaction with one’s social relationships. Most research has focused on the experience of loneliness in old age, but levels of loneliness are also known to be high among teenagers and young adults. While poor health may be associated with increased feelings of loneliness, there is now considerable evidence on the role of loneliness as a risk factor for poor mental and physical health. Studies suggest that loneliness is associated with an increased risk of developing dementia and chronic diseases, and also with a higher rate of Mortality. Risky Health Behaviors, a poor cardiovascular profile, and compromised immune functioning have all been proposed as potential pathways through which loneliness may affect health. However, much still remains to be understood about these Mechanisms. This bibliography starts with a section on chapters and reviews that provide a theoretical basis for understanding the associations between social relationships and health, as well as some reviews of the Health Effects of loneliness. This is followed by sections that focus on specific outcomes, health effects (mortality, general health and physical function, cognitive function and Mental Health), and Healthcare Utilization. The subsequent section examines research into the pathways through which loneliness affects health. The final section examines interventions to reduce loneliness and the associated effects on health outcomes.