Cancer stem cells (CSCs) are cancer cells that possess characteristics associated with normal stem cells, specifically the ability to give rise to all cell types found in a particular cancer sample. CSCs are therefore tumorigenic (tumor-forming), perhaps in contrast to other non-tumorigenic cancer cells. CSCs may generate tumors through the stem cell processes of self-renewal and differentiation into multiple cell types. This book presents new research on cancer stem cells including mesenchymal stem cells and their role in tumor progression; Wnt signaling in colon cancer stem cells; genetic and epigenetic alterations that drive leukemic stem cell self-renewal and others.