Gene-expression profiling of breast cancers has shown that distinct molecular subclasses are present within tumors that are apparently morphologically similar. The molecular subclasses of cohorts classified by the 'intrinsic' gene set include the luminal A and B, erbB-2+, normal-breast-like, and basal-like tumors. Basal-like breast cancers have been reported to be associated with worse overall and disease-free survival compared with the luminal A subtype. In addition, there is an immunohistochemical surrogate for the basal-like profile, which has considerably facilitated their study in non-specialty laboratories. Basal-like breast carcinomas have markedly reduced expression of genes related to estrogen receptors and erbB-2, and express proteins that are characteristic of the normal myoepithelial cell. This Review appraises the current state of knowledge on the clinical and pathologic features of breast cancers classified as 'basal-like' by gene-expression profiling and/or immunohistochemical criteria. These tumors seem to be relatively heterogeneous according to a multitude of clinicopathologic parameters, which indicates that their most prognostically relevant subsets have yet to be defined. Similarly to tumors of luminal epithelial differentiation, carcinomas of the 'basal' type have a spectrum of morphologic and clinical characteristics.