Several papers dealing with the clinical relevance of steroid hormone receptors in breast cancer have already been published. However, no publication has overviewed studies currently being conducted in Europe, nor is there a register of the european centers performing receptor assays. It has been our purpose to fill these gaps. A large majority of authors who we contacted kindly agreed to con tribute to our book. Papers were grouped into six main parts according to their contents. The abundance of clinical data relevant to receptor assays led us to add critical summaries to help the reader to form his own opinions on the subject. We would like to thank Drs. R. J. B. King, G. Contesso, L. Santi, and E. Engelsman, who helped us in this regard. We hope that this book will enable the reader to become acquainted with the present state of European steroid hormone receptor studies devoted to breast cancer treatment. We also hope that it will promote cooperation between European oncologists involved in this field of research.