This chapter focuses on primary and metastatic tumors involving the central and peripheral nervous systems as well as neurological complications of cancer and cancer treatments. The World Health Organization (WHO) Classification of Tumors of the Central Nervous System is the international standard for the classification of brain and spinal cord tumors, and the fifth edition (the WHO CNS5, published in 2021) is the most recent iteration. Historically, nervous system neoplasms were classified according to their histology, but molecular features are being incorporated into the diagnostic criteria for these tumors with increasing frequency, as molecular features are diagnostically, prognostically, and therapeutically important. Nervous system tumors affect both adults and children and can have a significant impact on both quality of life and lifespan. Further, systemic cancers and cancer-directed therapies can have a significant effect on neurological health and must be recognized and treated accordingly.