Several different types of tumors, benign and malignant, have been identified in the central nervous system (CNS). The prognoses for these tumors are related to several factors, such as the age of the patient and the location and histology of the tumor. In adults, about half of all CNS tumors are malignant, whereas in pediatric patients, more than 75% are malignant. For most benign CNS tumors that require treatment, neurosurgeons can offer curative resections or at least provide significant relief from mass effect. Unfortunately, we still lack effective treatments for most primary and secondary malignant CNS tumors. However, the past decade has witnessed an explosion in the understanding of the early molecular events in malignant primary CNS tumors, and for the first time in history, oncologists are seeing that a plethora of new therapies targeting these molecular events are being tested in clinical trials. There is hope on the horizon for the fight against these deadly tumors. The distribution of CNS tumors by location has remained constant for numerous years. The majority of primary CNS tumors arise in the major cortical lobes. Twenty nine percent of primary CNS tumors arise from the dural meninges that encase the CNS structures. The vast majority of these are meningiomas, of which over 90% are benign. About 10% of primary CNS tumors are found in the sella turcica region, where the pituitary gland resides. Other much less common sites of primary CNS tumors include the pineal region, ventricular system, cerebellum, brain stem, cranial nerves, and spinal cord. The distribution of CNS tumors by histology has seen a slight increase in more malignant tumors over the past decade, possibly due to increased neuroimaging practices or environmental exposures. Arising from glial cells, gliomas represent over 36% of all primary CNS tumors and consist of astrocytomas, oligodendrogliomas, ependymomas, mixed gliomas, and neuroepithelial tumors. The benign meningiomas make up 32% of primary CNS tumors, followed by nerve sheath tumors and pituitary tumors. Primary CNS lymphomas, embryonal tumors, and craniopharyngiomas are uncommon. The most common gliomas are astrocytomas, and these tumors are typically classified by the World Health Organization (WHO) as Grades I through IV. Grade IV, the most malignant grade of astrocytoma, includes glioblastoma multiforme (GBM), the most common malignant primary CNS glioma in adults, which represents 51% of all primary CNS gliomas. GBM is unfortunately the most challenging to effectively treat and has the worst patient survival. This chapter is therefore primarily devoted to the current understanding of this topic. Here we describe the molecular and cellular events associated with malignant glioma initiation and progression. We also review the importance of glioma stem cell biology and tumor immunology in early gliomagenesis. In addition, we present a brief description of the most common malignant primary CNS glioma in pediatric patients – medulloblastoma, as well as familial cancer syndromes that include gliomas as part of the syndrome.