(A) Tumors can arise from somatic cells through genetic mutations of cancer-critical genes. In addition, dysregulation of microenvironmental factors can contribute to the carcinogenic process. Such events might predominantly affect long-lived somatic stem cells, which can represent the cancer cell of origin, for example in mouse models of colorectal cancer (48). However, the CSC definition does not imply a specific relationship between CSCs and physiological stem cells. Findings in other disease models support progenitors or terminally differentiated somatic cell types as the source of malignant transformation. (B) CSCs are posited to be exclusively capable of driving tumorigenesis through 3 defining features: (i) their ability for long-term self-renewal, (ii) their capacity to differentiate into tumor bulk populations devoid of CSC characteristics, and (iii) their unlimited potential for proliferation and tumorigenic growth. Furthermore, CSCs in certain malignancies possess the capacity to drive tumor angiogenic responses and/or to engage in vasculogenic mimicry, potential means of promoting tumor growth. In addition, immunoevasive features of CSCs might contribute to tumorigenesis and ultimately to tumor progression. (C) CSCs can exhibit increased resistance to chemotherapeutic agents and/or ionizing radiation. CSCs might also possess a preferential capacity to withstand immune-mediated rejection. If CSCs indeed represent the pool of resistant cells in human cancer patients, they likely also drive neoplastic progression, tumor recurrence, and metastasis. Although this hypothesis requires further validation, clinical tumor progression has already been correlated with CSC frequency in human melanoma patients.