Monoclonal antibodies (MAs) are complex biotherapeutics as their molecular mechanism of action depends on multiple domains. Consequently regulatory approval of biosimilars of MAs is subjected to specific, science-based guidelines. An extensive comparative in vitro characterization to evaluate the biosimilarity of the various functional domains is required. The exquisite species specificity of MAs precludes reliable in vivo non-clinical evaluations and means that adequately designed clinical studies are extremely critical to confirm the biosimilarity. To date no biosimilar MAs have been approved. Taking into account the expected high development costs for biosimilar MAs, their use may well be superseded by alternative antibody formats and next-generation MAs.