Introduction: Obesity is defined as an abnormal or excessive build-up of body fat that may be harmful to health. Body mass index is most commonly used for diagnosing obesity. External factors such as diet, physical activity and metabolic state significantly contribute to obesity. General rule states that excessive body fat is the result of a long-term imbalance between the input and output of energy. Physical activity plays an important part in the physical, psychosocial and mental development of children. It also significantly affects the development of motor skills that are utilized especially in adulthood. Frequently referenced benefits of adolescents' physical activity include prevention of obesity, improvement of skeletal health, strengthening of heart and lung function as well as better psychosocial health. Ideally, all adolescents would adopt a healthy lifestyle in their childhood and continue with physical activity as adults. Purpose: The purpose of the bachelor's thesis is to determine, through a literature review of foreign and Slovenian articles and papers, the effect of the physical activity on lowering the body mass index in overweight adolescents. Methods: Databases PubMed, Google Scholar and PEDro were used to obtain suitable literature. We limited the electronic database search to hits that included full text. Inclusion factors were: healthy, obese adolescents, intervention through physical activity, publicly accessible articles. Exclusion factor was: adolescents with health issues. We used literature published between 2012 and 2018. Results: 32 pieces of research were reviewed in the course of literature review, of which five were chosen for the bachelor’s thesis. Main results indicate a decrease in BMI when using combined exercise compared to aerobic exercise or resistance exercise. Resistance exercise yields results considering the lowering of body mass index than aerobic exercise. Discussion and Conclusion: Physical activity can have different effects on the reduction of excess body weight and the lowering of body mass index, as effects depend on the type of physical activity, its intensity, and frequency. Evidence suggests that any physical activity has positive effects on health and body composition. Adding physical activity in the form of exercise, such as aerobic or resistance exercise, can contribute to even better results regarding losing weight and lowering of body mass index. Combined exercise gives the best results, but it is important that time and intensity of training are not excessive, as this may lead to a lack of motivation to persist. In children and adolescents the influence of parents and school is important as they need to set the right example and provide opportunities and motivation for a healthy and active lifestyle.