© 2014 by John Wiley & Sons, Inc. All rights reserved. Cancer stem cells (CSC) comprise a specialized neoplastic population and are responsible for maintaining tumors in several cancers. Cancer stem cells, like normal stem cells, can undergo processes modulating self-renewal and differentiation or commitment. Relapse in treated oncology patients is thought to arise from a pool of resistant cells, and this residual cell population has key significance because it is responsible for disease re-initiation. If the CSC constitutes this resistant population, they should be the dominant population in minimal residual disease, and it is critical to study them.