In childhood and young adulthood, a wide variety of brain tumors, such as medulloblastoma and ependymoma, frequently occur. Moreover, high- or low-grade diffuse gliomas, commonly found in adults, also emerge. Recent genomic research has revealed numerous molecular and genetic features of pediatric brain tumors. These molecular and genetic findings have been incorporated into the latest 2021 World Health Organization Classification of Tumors of the Central Nervous System(WHO CNS 5). WHO CNS 5 introduces separate classifications for adult- and pediatric-type diffuse gliomas, which were conventionally diagnosed using the same criteria. Classifying these adult- and pediatric-type gliomas using histopathological properties alone is challenging. Therefore, molecular diagnostics utilizing diverse molecular and genetic information, including variants, copy number alterations, structural abnormalities, and DNA methylation profiles, are imperative. Many molecular and genetic characteristics have been elucidated in the WHO CNS 5. Molecular diagnostics and classification are essential for accurately categorizing pediatric brain tumors, and the significance of molecular and genetic information will continue to grow.