Diffuse WHO grade II gliomas are histologically and genetically heterogeneous. The 2016 WHO classification redefines grade II gliomas with respect to morphological and molecular tumour alterations: grade II oligodendrogliomas are defined by the presence of whole-arm codeletion in chromosomal arms 1p/19q, whereas isocitrate dehydrogenase (IDH) mutations define subclasses of astrocytoma. Although histological grade remains useful, the prognoses of patients with glioma are more tightly associated with molecular alterations than with grade, and chromosomal and gene array technologies are becoming increasingly beneficial in understanding tumour genetic heterogeneity. The indolent nature of the disease often creates subtle neurological symptoms that can be overlooked or misunderstood, resulting in delayed diagnosis. Seizures often herald the diagnosis, especially in patients who have IDH mutations, which are associated with an increased production of 2-hydroxyglutarate. Treatment paradigms have shifted, owing to new diagnostic criteria and new clinical trial evidence. Patients benefit more from chemoradiation than radiation alone, especially those with tumour IDH1 Arg132His mutations; gross total resection of the tumour, including tumours with IDH mutations, is associated with prolonged survival. Initial observation remains appropriate in patients whose rate of disease growth is not yet completely defined; such patients could include those with completely resected disease and those with 1p/19q codeleted tumours.