The theoretical starting point of this paper is based on the fact that obesity in children, especially boys of early and preschool age, is a serious health problem that can have long-term physical and psychological consequences. The causes of obesity are complex and include genetic, environmental and social factors, but a reduced level of physical activity is one of the main factors. The basic theoretical backbone of the work starts from the connection between the level of physical activity and overweight and relies on scientific recommendations that indicate the importance of regular physical activity in the prevention of obesity. The aim of the paper is to analyze the influence of physical activity on obesity in boys of early and preschool age, to investigate the factors that influence the level of physical activity, and to present examples of good practice and successful obesity prevention programs in Croatia. The paper also aims to offer recommendations for increasing physical activity as a preventive measure against obesity, with an emphasis on the role of parents, preschool institutions and local communities in this process. The most important results of the work show that physical activity plays a key role in reducing the risk of obesity in preschool boys. Research confirms a positive association between higher levels of physical activity and lower rates of overweight. The paper also presents factors that influence the level of physical activity, including the role of parents, educational institutions and the socioeconomic environment. Through the analysis of successful preventive programs in Croatia, such as the "Healthy Living" and "Five" programs, the importance of integrated approaches that include education, physical activity and the promotion of healthy eating habits has been highlighted. Prevention of obesity through physical activity is essential for the healthy development of children. The role of parents, preschool institutions and local communities is necessary to create a stimulating environment for physical activity and the adoption of healthy habits. Successful prevention programs in Croatia show that timely measures and education can reduce the prevalence of obesity and improve the health of the child population.