The field of the cancer stem cell in solid tumors, such as breast, is in its infancy. Yet, the literature is rife with instances of hypothesis becoming dogma without critical examination of the data used to support the hypothesis of alternative explanations for the observations made. The aim of Stem Cells and Breast Cancer is to provide the readers with food for thought. Various contributors were selected who are ‘thought leaders’ in the area of stem cells in general and/or mammary stem cells in particular, to guide the reader in exploring and grasping the concepts necessary for a sound approach to the problem of identification and characterization of the cancer stem cell in the breast. The authors of the various papers contained in this work were asked to be provocative and to force the readers out of their comfort zone of thinking about stem cells in the breast and in breast cancer. They were also encouraged to question the current ‘dogma’ on what constitutes a stem cell and how one can recognize it and understand its properties. The articles in this publication explore fundamental questions of the origin, evolution and properties of the stem cell in normal as well as cancerous tissue. By understanding the fundamentals of the cancer stem cell and its niche hopefully better therapies can and will be designed.