Brain tumors are classified depending on the exact site of the tumor, the type of tissue involved, whether they are noncancerous (benign) or cancerous (malignant), and other factors. Sometimes, tumors that start out being less invasive can become more invasive.

Tumors may occur at any age, but many types of tumors are most common in a certain age group. In adults, gliomas and meningiomas are most common.

Today, most medical institutions use the World Health Organization (WHO) classification system to identify brain tumors.  Some tumor types are assigned a grade, ranging from Grade I (least malignant) to Grade IV (most malignant), which signifies the rate of growth. There are variations in grading systems, depending on the tumor type. The classification and grade of an individual tumor help predict its likely behavior.