The prevalence of obesity increases rapidly, in all ages and in all social classes. Epidemiological studies have documented that obesity have large long-term consequences, even in children, including development of metabolic syndrome and other obesity-associated comorbidities (i.e. diabetes mellitus type 2, cancer and cardiovascular diseases) which may largely impact the person's health, life expectancy, and quality of life. As summarised in this review, epidemiological studies add to our knowledge on comorbidities associated with obesity, the prognosis, and the prevalence of obesity over time.