Axillary lymph node metastases dramatically worsen the prognosis of patients with breast cancer. Despite this prognostic significance, routine histologic examination of axillary lymph nodes examines less than 1% of the submitted material. It is therefore obvious that micrometastatic disease is missed with this rather cursory examination, and the question arises as to the significance of this missed disease. Most lines of evidence suggest that missed axillary micrometastases exist and contribute to patient mortality. Most large studies of breast cancer micrometastases have suggested that undetected axillary micrometastases can be identified with more detailed examinations of the regional lymph nodes and that this group of patients has a poorer prognosis than those with no metastases identified. In addition, small-volume nodal disease, too small to be detected by traditional hematoxylin and eosin staining, has been shown to be capable of producing tumors in animal models. Finally, micrometastases have been shown to be of significance in other diseases. This article reviews the lines of evidence and the ongoing studies that are attempting to clarify the significance of micrometastatic disease in patients with breast cancer.