||Several tumor entities including brain tumors aberrantly overexpress intermediate conductance Ca2+ activated KCa3.1 K+ channels. These channels contribute significantly to the transformed phenotype of the tumor cells. By modulating membrane potential, cell volume, Ca2+ signals and the respiration chain, KCa3.1 channels in both, plasma and inner mitochondrial membrane, have been demonstrated to regulate many cellular processes such as migration and tissue invasion, metastasis, cell cycle progression, oxygen consumption and metabolism, DNA damage response and cell death of cancer cells. Moreover, KCa3.1 channels have been shown to crucially contribute to resistance against radiotherapy suggesting KCa3.1 channels as promising new targets of future anti-cancer therapies. The present article summarizes our current knowledge of the molecular signaling upstream and downstream and the effector functions of KCa3.1 channel activity in tumor cells in general and in glioblastoma cells in particular. In addition, it presents original in vitro data on KCa3.1 channel expression in subtypes of glioblastoma stem(-like) cells proposing KCa3.1 as marker for the mesenchymal subgroup of cancer stem cells. Moreover, the data suggest that KCa3.1 contributes to the therapy resistance of mesenchymal glioblastoma stem cells.