Body Mass Index, commonly known by the acronym BMI, is an international statistical measure used to assess the relationship between an individual's weight and height, serving as an indicator of weight adequacy. Developed in the 19th century by the Belgian mathematician Adolphe Quetelet, BMI is calculated using a simple formula: divide the person's weight (in kilograms) by the square of their height (in meters). The formula is $BMI = weight / (height²)$. The resulting number is then compared to classification ranges established by the World Health Organization (WHO), which generally include: Underweight (BMI < 18.5), Normal weight (18.5–24.9), Overweight (25–29.9), and Obesity (BMI ≥ 30), the latter often subdivided into Classes I, II, and III. Although BMI is a very accessible and widely used screening tool to identify potential health risks associated with underweight or excess weight at a population level, it has significant limitations. The main criticism is that BMI does not distinguish body composition, meaning it does not differentiate between fat mass and muscle mass. This means that very muscular individuals, such as athletes, may be incorrectly classified as overweight or obese. Similarly, it may underestimate body fat in older adults who have lost muscle mass. BMI also does not account for fat distribution (visceral vs. subcutaneous fat), bone density, or ethnic variations in body composition. For these reasons, BMI should not be used as an isolated diagnostic tool, but rather as an initial indicator that should be supplemented by other measurements, such as waist circumference, and a complete clinical evaluation to determine an individual's true health status.