Many epidemiological studies have shown that chronic metabolic diseases such as hypertension, diabetes, and cardiovascular disease (CVD) are more common in overweight/obese patients than in normal weight individuals. Accordingly, several clinical tools have been developed to assess body fatness in clinical practice, such as body mass index (BMI), skinfold measurement, bioelectrical impedance, hydrostatic weighing, and air-displacement plethysmography. The most commonly used index of relative body weight is BMI (i.e., the ratio of weight to height in kg/m2), and several cut-off values have been proposed in order to classify underweight, normal weight, overweight, and obesity. However, obesity is heterogeneous in its etiology and metabolic complications. In this regard, remarkable clinical observations made over 60 years ago drew attention to the fact that body fat distribution was more important than excess weight per se in obesity-related complications. These pioneering observations were later supported by findings from a wealth of studies. For instance, several prospective studies have shown that large amounts of upper body fat (abdominal obesity) increase the risk of type 2 diabetes, CVD, and attendant mortality. These prospective studies have all used simple anthropometric measurements such as waist circumference, waist-to-hip ratio, or sagittal diameter to estimate the absolute or relative amount of abdominal fat. It has been suggested that in order to assess the health hazard of overweight/obesity, a measure of overall adiposity (BMI) is required. However, it is also vitally important to take into account the presence of abdominal fat, which can be quantified in terms of waist circumference. Studies have shown that an elevated waist circumference can predict an increased risk of complications beyond simple BMI. Lastly, since the relationship of overweight/obesity to metabolic complications that increase the risk of type 2 diabetes and CVD varies based on ethnicity and sex, it has been proposed that sex- and ethnicity-specific waist cut-offs would make it easier to identify high-risk abdominally obese patients in all populations worldwide.