Radiotherapy in children can lead to early and late effects, influenced by radiobiology and tissue features. The Linear Quadratic model explains cell kill, with early effects predominantly seen in high-turnover tissues like skin and mucosa (high ɑ/β ratio) and late effects in differentiated tissues like neural structures (low ɑ/β ratio). Radiation side effects are categorized as stochastic (e.g., second malignancies) or deterministic (e.g., cataracts), with deterministic effects having dose thresholds. Quantifying normal tissue toxicity has evolved from expert opinion-based TD 5/5 constraints to detailed dose-volume data from QUANTEC for adults and ongoing PENTEC studies for pediatric cases. Common early effects include skin reactions, oral mucositis, esophagitis, myelosuppression, and transient brain injury. Late effects involve radiation necrosis, neurocognitive dysfunction, cerebrovascular events, optic neuropathy, sensorineural hearing loss, and cranial neuropathy. Neurocognitive decline is particularly significant in children, with IQ reductions linked to higher cranial radiotherapy doses. Techniques like hippocampal sparing and memantine show promise in adults but lack pediatric data. Other late toxicities include also include cerebrovascular events, optic neuropathy, infertility and second malignancies. These findings underscore the need for tailored radiotherapy approaches for pediatric patients.