Excess body mass and obesity have become serious public health problems in the world, and in Croatia as well. It is worrying to what extent obesity appears in children and adolescents. Improper nutrition and little or no physical activity greatly affect the health and quality of life of each individual. In addition, obesity can be the cause of various diseases, disorders and incapacity for the normal functioning of the human body. Parents have the greatest influence on a child's growth and development. Whatever habits a child acquires at an early age, there are great chances that he will have such habits when he grows up. Parents should provide their children with a varied diet and encourage them to exercise daily. Physical and health education classes at school are certainly not enough, so it would be desirable if the child were involved in extracurricular and extracurricular sports activities. The paper explains the method of determining obesity, explains the types of obesity and states the state of obesity in the world and in Croatia in the past few years. The types of obesity prevention are indicated and their programs are described. Afterwards, proper nutrition and the influence of physical activity on children's development and health are described. Research was conducted on 53 first- and fourth-grade students of the Nikola Tesla Elementary School. The aim of the research was to determine whether there is obesity among female students in primary education and whether age affects the level of obesity. We measured the height (cm) and weight (kg) of the students and obtained the body mass index (BMI) with which we determined whether the child was obese or not. We also conducted a survey with questions about eating habits and the amount and type of physical activity.