It is widely debated whether all tumor cells in mammary tumors have the same potential to propagate and maintain tumor growth or whether there is a hierarchical organization. Evidence for the latter theory is mainly based on the ability or failure of transplanted tumor cells to produce detectable tumors in mice with compromised immune systems; however, this assay has lately been disputed to accurately reflect cell behavior in unperturbed tumors. Lineage tracing experiments have recently shown the existence of a small population of cells, referred to as cancer stem cells (CSCs), that maintains and provides growth of squamous skin tumors and intestinal adenomas. However, the lineage tracing techniques used in these studies provide static images and lack the ability to study whether stem cell properties can be obtained or lost, a process referred to as stem cell plasticity. Here, by intravital lineage tracing, we report for the first time the existence of CSCs in unperturbed mammary tumors and demonstrate CSC plasticity. Our data indicate that existing CSCs disappear and new CSCs form during mammary tumor growth, illustrating the dynamic nature of these cells.