Obesity is a chronic disease that has taken on the characteristics of a global pandemic over the last ten years, both among adults and children. Overnutrition and obesity are one of the most significant public health problem that require preventive and therapeutic interventions and more controls. Obesity is characterized by an excessive amount of body fat in the total body mass, which is dangerous for an individual's health. Statistical data in the thesis shows constantly growing number of overnourished people in the world, Europe and Croatia, classification and diagnosis of obesity, factors that cause obesity and its impact on the physical and psychological state of the individual. The research analyzed the prevalence of overweight among 3rd grade elementary school students in Sisak. Anthropometric measurements of the height and weight of students were carried out, and the body mass index (BMI) was calculated from these data. The goal of this research is to determine the stage of prevalence of overweight and obesity among 3rd grade elementary school students, analyze the data and compare it with nutritional age categories according to CDC (Centers for Disease Control and Prevention) criteria. 47 students took part in the research, of which 24 were boys and 23 were girls. Students in the sample are between 9.0 and 10.0 years old. The results of the research show that 63,8% students are in the level of normal nutrition, which is lower than expected level of 80%. 17.0% of students are overweight and 12.8% are obese. For optimal growth and development, children need to be provided with a healthy and nutritious diet along with a sufficient amount of physical activity. The obesity prevention program should educate children and parents about creating healthy habits and the importance of physical activity.