Several studies have provided ample evidence of a clinically significant interobserver variation of the histological diagnosis of glioma. This interobserver variation has an effect on both the typing and grading of glial tumors. Since treatment decisions are based on histological diagnosis and grading, this affects patient care: erroneous classification and grading may result in both over- and undertreatment. In particular, the radiotherapy dosage and the use of chemotherapy are affected by tumor grade and lineage. It also affects the conduct and interpretation of clinical trials on glioma, in particular of studies into grade II and grade III gliomas. Although trials with central pathology review prior to inclusion will result in a more homogeneous patient population, the interpretation and external validity of such trials are still affected by this, and the question whether results of such trials can be generalized to patients diagnosed and treated elsewhere remains to be answered. Although molecular classification may help in typing and grading tumors, as of today this is still in its infancy and unlikely to completely replace histological classification. Routine pathology review in everyday clinical practice should be considered. More objective histological criteria for the grade and lineage of gliomas are urgently needed.