Doctor Jekyll and Mister Hyde: Dual nature of stem cells in cancer The existence of stem cells in cancer have already been perceived by Greek physician Hipokrates, but the scientifically based idea on cancer stem cells was first coined by pathologist Rudolf Virchow at the end of 19th century. The biotechnological development and today’s modern insights into molecular networks of cancer cells functioning, have contributed to the notion on hierarchical concept of tumour progression, driven by cancer stem cells and in addition to better understanding of their cross-talk with neighboring non-malignant cells within the tumour. Cancer stem cells are triggering the opposed activties by normal, mesenchymal stem cells, aiming at killing Mr Hydea)* in the body of Dr. Jekylla, who is inventing a potion against his own preveritve nature. Cancer stem cells have been recognized at the beginning of this century also in malignant brain tumour glioblastoma. Due to their unusual characteristics and plasticity, both in some aspects not in line with principles of classical cell biology, they are often debated and even their existence is denied. In this lecture, our recent findings on glioblastoma stem cells and their dialogue with mesenchymal stem cells that can lead to a more or a less aggressive tumor, will be presented. However, how to influence the fight between bad and good is of course not only the problem of curing cancer!