WHO stands for World Health Organization, the entity which developed the widely used and accepted classification system for brain tumors. Brain tumors are graded from low grade (grade I) to high grade (grade IV). The grade of a tumor refers to the way the cells look under a microscope. Cells from higher grade tumors are more abnormal looking and generally grow faster than cells from lower grade tumors; higher grade tumors are more malignant than lower grade tumors. Higher grade tumors may also be referred to as anaplastic, for example, an Anaplastic Astrocytoma is a grade III tumor. Because they are located in the brain, it is important to recognize that even a low grade brain tumor can be dangerous because, as a tumor in the brain grows, it presses upon normal brain tissues which can cause inflammation and brain swelling. It is therefore very important that both low and high grade tumors be treated as early as possible, even if the treatment is only serial MRIs for low grade tumors.