Malignant gliomas are common primary tumors of the central nervous system, characterized by aggressive cell proliferation, diffuse infiltration and resistance to conventional therapy. Glioblastoma (former Glioblastoma multiforme, GBM), grade IV astrocytoma, is the most aggressive tumor, with a median survival of around 14 months. New therapies against this devastating and invariably fatal disease are needed. Stem-like cell populations have been identified in a number of malignancies including glioblastoma. These rare stem cells (called also glioma-initiating cells) are believed to be responsible not only for tumor initiation and progression but also resistance to therapeutic agents and tumor recurrence. Recently, the population of cells within glioblastoma with stem-like properties has gained increasing attention as a target to refine treatment strategies. This chapter aims to summarize the recent data regarding isolation, biology and mechanisms of resistance of glioblastoma stem-like cells to therapy.