The prevalence of obesity and associated chronic diseases has increased rapidly in Taiwan. Data from three consecutive Nutrition and Health surveys in Taiwan show that obesity prevalence has tripled for elementary school boys and doubled for girls since 1993–1996. About one-third of boys (15.5% and 14.7% for overweight and obesity, respectively) and one-quarter of girls were either overweight (14.4%) or obese (9.1%) in 2001–2002. For adults, obesity prevalence rates defined by body mass index ≥27 kg m−2 increased from 10.5% in men and 13.2% in women in the 1993–1996 survey, to around 17% in 2005. Prevalence of overweight was around 20% in 1993–1996 for both men or women, and increased to 30% in 2005 for men. No change was found in women. The underprivileged regions usually had higher prevalence of obesity and associated diseases. Scientific bases for Taiwan obesity definition are set out together with the screening and management plans. High-calorie intake was associated with obesity in young children (grades 1–2), but not in older children and adults. Physical inactivity and sedentary lifestyle-related variables were associated with obesity in men and older boys. In addition, good dietary quality was associated with a lower risk of obesity independent of energy intake in elderly Taiwanese. More research is needed to find effective determinants and public health measures for obesity, and concerted efforts are required to combat this rising health problem.