The current World Health Organisation (WHO) Classification of Central Nervous System Tumours defines oligodendrogliomas by IDH mutation and 1p/19q codeletion. Oligodendrogliomas differ from diffuse astrocytomas regarding genetic alterations in telomere maintenance mechanisms by frequently displaying TERT promoter mutations while astrocytomas typically exhibit ATRX (a-thalassaemia/mental retardation syndrome X-linked) mutations leading to alternative lengthening of telomeres (ALT). The distinction between both glioma types is crucial since it has a considerable impact on both patient treatment and outcome.