This publication is dedicated to the sentinel node (SN) procedure in breast cancer, and more specifically to the treatment of patients in whom isolated tumor cells or micro metastases are detected. The sentinel node procedure is based on the premise that if the first node the breast tissue drains into is clean, the remaining axillary lymph nodes are not likely to be involved and their removal will not be necessary. The introduction of this procedure during the 1990s meant a reduction in overtreatment for many patients, and therefore a reduction in morbidities such as lymph edema and shoulder dysfunction. There have been some concerns, however, which have led to an intensified SN pathology protocol. These concerns, together with an increase in the detection frequency of small nodal metastases and questions about whether the presence of isolated tumor cells or micro metastases is associated with overall breast cancer outcomes, mean that the SN procedure is still the subject of much research and discussion. The results of a number of randomized studies are expected within the next few years. This book is a collection of six published studies related to the prediction of non-SN involvement. These studies show that not only the size of SN involvement but also primary tumor characteristics play a role and that it is of pivotal importance to estimate non-SN involvement for each individual patient. By providing an overview of current evidence, this book will be supportive in making the best decision in current patient care.