Recent findings in the field of molecular-genetic alterations in diffuse gliomas, encompassing both adult and pediatric types, have initiated significant changes in their classification and diagnostics. These changes are presented in the fifth edition of the WHO Classification of Tumors of the Central Nervous System emphasizing a pivotal shift from the conventional categorization of tumors based on morphology to an integrated diagnostic approach, which incorporates characteristic molecular-genetic and epigenetic alterations alongside traditional morphological features. This review presents a summary of the units that are included in the group of diffuse gliomas of the adult type, with an emphasis on diagnostic criteria and grading according to the fifth edition of the WHO Classification of Tumors of the Central Nervous System, published in 2021.