Research into the relationship between happiness and health is developing rapidly, exploring the possibility that impaired happiness is not only a consequence of ill-health but also a potential contributor to disease risk. Happiness encompasses several constructs, including affective well-being (feelings of joy and pleasure), eudaimonic well-being (sense of meaning and purpose in life), and evaluative well-being (life satisfaction). Happiness is generally associated with reduced mortality in prospective observational studies, albeit with several discrepant results. Confounding and reverse causation are major concerns. Associations with morbidity and disease prognosis have also been identified for a limited range of health conditions. The mechanisms potentially linking happiness with health include lifestyle factors, such as physical activity and dietary choice, and biological processes, involving neuroendocrine, inflammatory, and metabolic pathways. Interventions have yet to demonstrate substantial, sustained improvements in subjective well-being or direct impact on physical health outcomes. Nevertheless, this field shows great potential, with the promise of establishing a favorable effect on population health.