With the introduction of sophisticated tools of molecular biology, prodrug activating gene therapies have evolved as a novel therapeutic option for high-grade malignant gliomas. Herpes simplex virus thymidine kinase/ganciclovir (HSV-tk/GCV) is an extensively studied form of cytotoxic gene therapies. It is especially applicable for localized cancers, such as malignant glioma because of its restricted anatomical location and absence of metastasis. The early successes in the treatment of experimental malignant gliomas in the 1990s, gave impetus to further test this approach in this devastating disease. In malignant glioma, the recurrence after conventional therapy is inevitable, due to the residual cells in the tumor bed. The fascinating feature of adenoviral HSV-tk is that it attacks the residual dividing tumor cells without affecting the non-dividing neurons and furthermore, exploits them to destroy the malignant cells via so-called bystander-effect. Clinical Phase I and II studies have shown significant survival advantage and excellent safety profile when compared to conventional treatments. Thus, the adenoviral mediated HSV-tk gene therapy is a promising new adjuvant treatment for patients with operable high-grade glioma.