In 1997, the PTEN gene (phosphatase and tensin homolog deleted on chromosome 10) was identified as a tumor suppressor gene on the long arm of chromosome 10. Since then, important progress has been made with respect to the understanding of the role of the Pten protein in the normal development of the brain as well as in the molecular pathogenesis of human gliomas. This review summarizes the current state of the art concerning the involvement of aberrant Pten function in the development of different biologic features of malignant gliomas, such as loss of cell-cycle control and uncontrolled cell proliferation, escape from apoptosis, brain invasion, and aberrant neoangiogenesis. Most of the tumor-suppressive properties of Pten are dependent on its lipid phosphatase activity, which inhibits the phosphatidylinositol-3'-kinase (PI3K)/Akt signaling pathway through dephosphorylation of phosphatidylinositol-(3,4,5)-triphosphate. The additional function of Pten as a dual-specificity protein phosphatase may also play a role in glioma pathogenesis. Besides the wealth of data elucidating the functional roles of Pten, recent studies suggest a diagnostic significance of PTEN gene alterations as a molecular marker for poor prognosis in anaplastic astrocytomas and anaplastic oligodendrogliomas. Furthermore, the possibility of selective targeting of PTEN mutant tumor cells by specific pharmacologic inhibitors of members of the Pten/PI3K/Akt pathway opens up new perspectives for a targeted molecular therapy of malignant gliomas.