This paper reviews definitions of obesity and evidence concerning risk factors for becoming obese. Modifiable aetiological factors, including diet and physical activity, are described, and possible interactions with genetic predisposition are addressed. The impact of obesity on health and quality of life is then considered. Although epidemiological studies have not observed linear associations between increasing weight-for-height and decreasing longevity, there is a consensus that excess mortality occurs at extremely high body mass index values. Compromised quality of life is also observed at this end of the obesity distribution, and is manifested as decreased psychological well-being, less social integration, and stigmatisation.