Cancer stem cells are undifferentiated cells capable of being transformed into all types of cells comprising the tumour mass. As demonstrated by recent findings, they are responsible for the development of both haematopoietic malignancies and solid lesions. Cancer stem cells determine the unlimited growth of tumours and the variety of their morphology; they are also suspected of being the factor responsible for metastases. Their capability for renewal is the cause of disease recurrences even after long remission periods. Studies have shown that cancer stem cells may develop from normal stem cells, mature cells, and differentiated, or already transformed cells. What differentiate these cells from other cancer cells are rare mitotic divisions, which play a protective role for the cancer genome.