Triple-negative breast cancer (TNBC) is a heterogeneous disease diagnosed by immunohistochemistry and is characterised by tumours that do not express estrogen receptor (ER) or progesterone receptor (PR) at all, and do not overexpress human epidermal growth factor receptor 2 (HER2). Prototypical TNBC is aggressive in nature and associated with a poor prognosis, making the accurate diagnosis of the disease vitally important for ensuring optimal therapy for patients. Morphological and biological analyses can identify subtypes of TNBC, which can have different prognoses, and (in the case of the latter) may eventually be used to predict response to treatment. This mini-review focuses on clinically relevant issues in the diagnosis of TNBC, including the importance of adherence to international guidelines for the detection of ER/PR/HER2 status, and the relationship between TNBC and the overlapping (yet distinct) intrinsic subtype of ‘basal-like’ breast cancer. In addition, we review the potential use of emerging biomarkers as surrogates for molecular subtypes and as a means of identifying potential responders to new therapies.