While biosimilars of low molecular-weight biologics such as G-CSF have been available in Europe since 2006, biosimilars of monoclonal antibodies (mAbs) have only become available in the last year. Unlike G-CSF, mAbs are large and complex and often play a direct role in the survival of patients with life-threatening illnesses such as breast cancer. Several biosimilars are currently under development for the treatment of breast cancer, and the use of biosimilars in a setting that directly impacts patient survival raises a number of questions. In this review, we discuss the biosimilar mAbs currently in development for the treatment of breast cancer. We provide an overview of the European Medicine Agency guidelines and historic data on the development of biosimilars in order to discuss the development of biosimilar mAbs for breast cancer. Biosimilars offer a highly attractive path toward reducing the cost of medical care and should be pursued with great interest. However, for agents used to treat life-threatening diseases such as cancer, a cautious approach must be taken to ensure that there is no negative impact on patient care. Clinical trials for biosimilar mAbs must be carried out in an appropriately sensitive patient population using endpoints that can accurately demonstrate both the similarity of the biosimilar and its efficacy in the indication. Due to the abbreviated approval pathway, rigorous pharmacovigilance must be in place once a biosimilar mAb is approved in order to ensure its long-term safety and efficacy.