Obesity is considered one of the chronic, non-contagious diseases, i.e. a civilization disease; it constitutes a threat to mankind’s health. To assess the degree of obesity, as well as to present the distribution of anthropometrical data, the following is used: centile charts, body mass index (BMI), Cole’s index, waist and hip size measurements, and skinfold calipers. Most frequently, the simple (ordinary) obesity is diagnosed, and the secondary much more seldom. With regard to the fat tissue localization, two types of obesity are distinguished: android and gynoid. Android obesity creates a high risk that a metabolic syndrome can develop. A positive energy balance that continues over time leads to the simple (ordinary) obesity, that is also supported by improper nutrition habits (sweet beverages, “fast food”, snacks), as do low physical activity and emotional problems. Obesity is multigene inherited, and the realization of tendency to excessive body mass depends on the impacts of environment. Obesity in children and young people generates a risk of the following diseases: respiratory system diseases, endocrine system diseases, fatty degeneration of the liver, orthopaedic malformations and psychiatric disorders, and later on: cardio-vascular diseases and type 2 diabetes. Treating obesity is closely related with the age and degree of overweight of obese patients. The basic purpose of therapy is to maintain or to reduce the patient’s weight, and to prevent medical complications resulting from their obesity.