We believe that this so called hierarchical (pyramidal) organization of our normal tissues and organs is conserved within tumor masses with cancer stem cells (CSCs) at the very top of this hierarchy. Likewise, the niche of CSCs is essential to determine their capacity to proliferate, invade surrounding tissues, and eventually colonize distant organ sites, the so-called metastases. By comparing normal and cancer stem cells and by studying the CSC niche, we hope to identify key molecular and cellular mechanisms to be specifically targeted in future therapies.