INTRODUCTION: Childhood obesity is a particular public health issue due to its negative impact on lifelong health. The aim of this doctoral dissertation was to investigate the association between school environments that provide children with access to proper nutrition and opportunities for physical activity, and the prevalence of overweight and obesity in second and third grade children in elementary schools in the Republic of Croatia. SAMPLE AND METHODS: The doctoral dissertation is part of the Childhood Obesity Surveillance Initiative research. The study was conducted in 182 second grades and 182 third grades from 164 elementary schools in the Republic of Croatia. RESULTS: At the national level, no association was found between the studied determinants of proper nutrition availability in schools and the average rates of overweight and obesity in children. In the Continental region, an association was found between the presence of a store or café within the school and the average rate of overweight children per school (F (5,80) = 5.794, p < 0.001, R2 = 0.266), controlling for the impact of local government unit development and mothers' socioeconomic characteristics. At both the national level and in the Continental region, statistically significant associations were found between the average rate of obese children in schools and the total opportunities for physical activity in schools. These associations were confirmed even after controlling for the development of local government units and mothers' socioeconomic determinants; nationally (F (6,139) = 3.630, p = 0.002, R2 = 0.135) and in the Continental region (F (6,139) = 3.630, p = 0.002, R2 = 0.135). In the Continental region, after controlling for the possible impact of socioeconomic variables, the existence of an outdoor school playground (B = - 18.130, 95% CI [- 28.793; - 7.467]) and conducting Physical and Health Education classes for 135 minutes per week (B = - 10.316, 95% CI [- 18.896; - 1.737]) were identified as statistically significant predictors of the average number of obese children per school. CONCLUSION: According to the obtained results, public health action is necessary to positively influence the development of healthy habits and reduce the prevalence of overweight and obesity in children.Obesity is a growing public health problem worldwide. Its consequences have a negative effect on all aspects of an individual's health, including physical, mental and social health.