This bachelor thesis focuses on oncological diseases in children and adolescents. It deals with differences between pediatric patients and adults, what are the causes of their malignant diseases, prevention, symptoms and treatment. In the practical part I focused on radiation therapy in children, using photon beam and proton beam, their advantages and disadvantages. I dealt with the incidence of the most common malignancies in children aged 0-19 years in the period 1996-2016. I analyzed the data from the National Cancer Registry and processed it into graphs and tables. I also looked at the data on the long-term survival of cured patients. The collected data show rather a decreasing tendency in the incidence of childhood malignancies in the monitored period 1996-2016. Tumor mortality in children has been decreasing in the period under review, after an initial sharp decline in recent years rather slightly. The higher incidence of malignancies in children aged 0-19 is in boys. I also graphically processed the age distribution of patients, where the first wave of increased incidence is recorded in the youngest patients under 4 years and the second wave in adolescents aged 15-19 years, which is different from the distribution of tumor incidence in the adult population where the risk of cancer increases with age. The relative percentage of five-year survival in pediatric cancer patients over time has increased with the development of cancer treatment and supportive care and dispenzarzation. The content of this work can serve as a basic overview of cancer of children and adolescents for health professionals and the general public.