Traditional cancer diagnostic techniques include assessment of histologic appearance, identification of specific tumor subtypes, tumor grading, assessment of lymph node status, and presence of metastasis. These are useful for initial evaluation, but are limited in their ability to predict response to treatment and/or risk of experiencing adverse events to particular treatments. Recently, there has been considerable progress in molecular diagnostics in this area. Using molecular-based technologies, it is now possible to identify some patients with an inherited risk of developing breast cancer, and to more accurately assess the prognosis of those who already have developed the disease. Various techniques are also available or under investigation to help select patients who are most likely to respond to certain types of treatment. At present, this includes diagnostic tests to identify single tumor markers for which specific therapies are available, such as estrogen receptor and HER2. Diagnostic systems that allow simultaneous assessment of multiple markers, such as DNA microarrays and proteomics, are currently being validated. The use of these systems will allow further development of customized therapies directed toward the particular molecular defects in a given cancer.