There is increasing evidence for the "cancer stem cell hypothesis" which holds that cancers originate in tissue stem cells or progenitor cells. As a result of this, cancers are driven by a cellular subcomponent that retains stem cell properties. Among these properties are self-renewal and multi-lineage differentiation. The biological processes which account for stem cell properties are currently being elucidated. Cancer stem cells maintain many of the same characteristics of their normal counterparts. The combination of biological research with mathematical modeling may provide for a greater understanding of the complex picture of breast cancer stem cells and assist cancer biologists and clinical oncologists in designing and testing novel therapeutic strategies.