Cancer can occur in any part of the brain or spinal cord. In 1997, about 18,000 new brain tumors were diagnosed, which was a 50% increase from only ten years before. They are rare tumors, representing only 1.5% of all cancers reported in the United States. The causes of central nervous system tumors are not known, and scientists cannot explain why brain tumors develop in healthy adults. Certain factors, however, have been identified that may increase a person's chance of developing a brain tumor. For example, workers in the oil refining, rubber manufacturing, and drug manufacturing industries have higher rates of certain types of brain tumors. Researchers are also studying families in whom multiple members have developed the same type of brain tumor to see whether heredity plays a role. They are also looking at the connection between viral infections and exposure to radiation and the development of brain tumors. There is no research to suggest that head injuries cause or increase a person's risk for developing a brain tumor. The use of older cell phones may be linked to brain cancer, but more study is needed to clarify this since some studies have not shown a higher risk. Because most patients diagnosed with a brain tumor have no identifiable risk factors, it is believed that brain tumors result from a number of factors acting together.

Tumors which start in the brain are called primary brain tumors and are classified according to the kind of cell from which the tumor seems to originate. The most common primary brain tumor in adults comes from cells in the brain called astrocytes that make up the blood-brain barrier and contribute to the nutrition of the central nervous system. These tumors are called gliomas (astrocytoma, anaplastic astrocytoma, or glioblastoma multiforme) and account for 65% of all primary central nervous system tumors.

Cancer from other parts of the body can spread to the brain and cause secondary tumors through a process called metastasis. Although it is possible for cancer from anywhere in the body to spread to the brain, it happens most often with cancers of the breast and lung. The cells of a metastatic brain tumor resemble the cells of the organ where the tumor started, not brain cells. For example, if a tumor starts in the breast and spreads to the brain, the cells of the brain tumor will resemble abnormal breast cells, not abnormal brain cells.