Neoplastic disease whether benign or malignant, is the aberrant uncontrolled proliferation of cells within any given compartment of the human body, leading to the formation of a tumour. Such proliferation of cells, known as transformation, is due to deregulation of cellular mechanisms that control normal growth and development, so that transformed cells become independent of factors that usually provide constraints for cell growth. Intracranial tumourigenesis essentially results in tumours of the brain, the meninges, the pituitary gland and/or the cranial nerves. (From Chapter 1)