Isocitrate dehydrogenase 1 (IDH1) gene mutations, primarily of the R132H type, occur in approximately 60 - 90% of diffuse and anaplastic gliomas and secondary glioblastomas. IDH mutations in gliomas are associated with several clinically relevant parameters including patient age, histopathological diagnosis, combined 1p/19q deletion, TP53 mutation, MGMT promoter hypermethylation and patient survival. Therefore, testing of the IDH status is relevant for diagnostic and prognostic considerations in primary brain tumors. IDH status can be assessed by immunohistochemistry or DNA-based methods including gene sequencing in the routine setting. Here, we review the relevance of IDH testing in diffuse gliomas and present practical instructions including detailed descriptions of procedures and protocols for diagnostic IDH testing using immunohistochemistry (for both automated and manual staining) and gene sequencing. Our article may provide guidance for laboratories aiming at establishing IDH testing for diagnostic evaluation of primary brain tumors.