The aim of this paper is to raise awareness of the growing problem of obesity in general and preschool children. The causes of obesity can be different, and the most common ones are poor eating habits and insufficient exercise. In order to estimate the amount of fat in the body of some people, a formula is used to calculate the BMI, or body mass index. Nowadays, when the number of obese children is higher than ever, monitoring the child's BMI is of great benefit. Obesity leaves a number of negative consequences on the organism of the child and there is a high chance that the excess weight will be maintained through adolescence and adulthood. Obesity brings health problems but also creates mental and psychological problems for the child. Children who struggle with being overweight often experience discrimination, deal with teasing and find it harder to have social relationships with peers. A healthy and optimal growth and development of a child requires a balanced diet and sufficient physical activity. The paper presents ways to prevent the development of childhood obesity in order to prevent the occurrence of long-term complications in adulthood. Also, studies on obesity and physical activity of preschool children are presented in this paper.