Gliomas, the most frequent primary brain tumors in humans, form a heterogeneous group, encompassing many different histological types and malignancy grades. Within this group, the diffuse infiltrative gliomas are by far the most common in adults. The major representatives in this subgroup are the diffuse astrocytic, oligodendroglial, and mixed oligo-astrocytic tumors. Especially in these diffuse gliomas, the role of molecular diagnostics is rapidly increasing. After summarizing the most relevant genetic aberrations and pathways in these tumors detected up till now, this review will discuss the clinical relevance of this information. Several molecular markers have been identified in diffuse gliomas that carry diagnostic and prognostic information. In addition, some of these and other markers predict the response of these gliomas to particular (chemo)therapeutic approaches. The techniques used to obtain this molecular information, as well as the advantages and disadvantages of the different techniques will be discussed. Finally, future perspectives will be presented with regard to the contribution of molecular diagnostics to tailor-made therapy in glioma patients.