The BMI was developed by a statistician as a method of determining obesity based on the height and weight of the individual. It does not take into account muscle mass, bone density, gender, or age, so all it does is provide rough measure of body composition. It has been an international standard for obesity measurement since the 1980’s and, accurate or not, the medical and insurance communities use it to qualify patients for weight loss surgery. Some medical professionals believe that your body fat percentage is a better way of judging your obesity and the attendant health risks. Even a "skinny" person can have high body fat percent (the fat can lurk in your viscera where no one can see it). The distribution of your fat is also important. In overweight people (BMI 25+), a high waist circumference is associated with an increased risk for type 2 diabetes, dyslipidemia, hypertension and cardiovascular disease. If you are overweight (BMI 25+), an unhealthy waist circumference is (as a general rule) above 35 inches for women, or above 40 inches for men.
A woman named Kate Harding has done a study of people of all shapes and sizes. The photographs (with BMI captions) on her site will make you think twice about whether BMI is a good measure of obesity. Check it out at: http://kateharding.net/bmi-illustrated/ .