Physical activity and proper nutrition are the most important factors in preventing obesity in children. In today's society, where the pace is more and more hectic and more time is spent in activities that do not require physical effort, it is necessary to develop in children the habit of moving as much as possible and engaging in physical activities. A hectic lifestyle also contributes to an improper diet because people rarely cook due to lack of time and eat more fast food that is easily available and ready to use quickly. The development of technology and a lifestyle based on sedentary activities also contribute to the development of obesity. A balanced and proper diet consisting of three main meals and two snacks is responsible for the children's normal growth and development because it supplies the body with all the necessary substances such as protein, carbohydrates, vitamins and minerals. The aim of this paper is to point out the problem of obesity and what leads to it, based on previous research. Various studies show that gender, age, motor skills and the environment affect the children's physical activity. Free time is mainly spent watching television and playing games with little time set aside for some physical activity. Also, for the purposes of this paper, research was conducted in the third grades of elementary school, in which the eating habits and physical activity of male and female students were examined. BMI was also calculated based on their weight and height.