The era of breast cancer as a single disease, and one-size-fits-all treatment, has passed. Hormone receptor status has allowed us to identify two phenotypic subtypes of breast cancer superimposed on the existing light microscopic histologic classifications, and in recent years HER2 has added a third axis of categorization. These three markers (ER, PR, and HER2) share the fact that they are linked inextricably to treatment decisions as the functional targets of specific agents and therefore to a degree the treatment has defined the disease. With the availability of molecular subtyping relying on mRNA in paraffin-embedded tissue or fresh-frozen material, precise assessments of gene loci for amplification, deletion, or mutation, and the development of high-throughput techniques, we now are at the beginning of an era when it may be possible and appropriate to generate a genetic profile for each patient's tumor such that we will subset breast cancer further and will tailor therapy for these subsets. Already two commercial tests are available to clinicians (Mammaprint and OncytopeDx), each offering prognostic information based on a collection of genes – and the latter also providing predictive information with regard to the value of chemotherapy. Each is currently being tested in prospective studies to provide additional information about how best to integrate them into routine care. On the horizon are newer and potentially more informative techniques, such as representational oligonucleotide microarray, a version of comparative genomic hybridization, which can provide more detail regarding gene amplifications and deletions. This presentation will review the available technologies and discuss their potential clinical utility.