The American Association of Clinical Endocrinologists (AACE) and the American College of Endocrinology (ACE) suggest that the diagnosis of obesity should not be centered solely on body mass index (BMI), but also on the evaluation of the presence of obesity-related complications, including metabolic syndrome, prediabetes, type 2 diabetes, dyslipidemia, hypertension, nonalcoholic fatty liver, polycystic ovary syndrome, sleep apnea, osteoarthritis, gastric esophageal reflux disease, and disability or inability to exercise. AACE and ACE propose a four-step approach to the diagnosis of obesity: initial screening based on BMI; clinical assessment of overweight and obese patients for complications; classification of obesity based on criteria related to complications; and treatment based on clinical judgment. According to the “four-step diagnostic method”, all people are classified into 5 levels: 1. normal weight (BMI <25 kg/m2); 2. overweight (BMI 25~29.9 kg/m2, no obesity-related complications); 3. grade 0 obesity (BMI ≥30 kg/m2, no obesity-related complications); 4. grade 1 obesity (BMI ≥25 kg/m2, no obesity-related complications); 5. grade 2 obesity (BMI ≥25 kg/m2, no obesity-related complications); 6. BMI ≥ 25 kg/m2 with at least 1 mild to moderate obesity-related complication); 5, Class 2 obesity (BMI ≥ 25 kg/m2 with at least 1 severe obesity-related complication). In some ethnic groups, BMI of 23-25 kg/m2 but with thickened waist circumference and at least 1 obesity-related complication also belong to grade 1~2 obesity. In terms of treatment, experts recommend: 1. Grade 0 obesity only requires lifestyle modification; 2. Grade 1 obesity requires intensive lifestyle and behavioral interventions, and medication can be considered; 3. Grade 2 obesity requires medication along with intensive lifestyle and behavioral interventions, and can be considered for weight loss surgery.