Malignant gliomas are among the deadliest primary brain tumors. Despite multimodal therapy and advances in chemotherapy, imaging, surgical and radiation techniques, these tumors remain virtually incurable. Glioma stem cells may be responsible for resistance to traditional therapies and tumor recurrence. Therefore, elimination of glioma stem cells may be crucial for achieving therapeutic efficacy. Metformin, a small molecule drug widely used in the therapy of type 2 diabetes, has shown significant anti-tumor effects in patients with breast cancer and prostate cancer. Recent preclinical data suggest that metformin also has therapeutic effects against glioma. Here we review the markers and hallmarks of glioma stem cells, and the molecular mechanisms involved in therapeutic targeting of glioma stem cells by metformin.