Biologicals are distinct from small molecule drugs in that they are larger, more structurally complex agents. While the overall risk is modest, the active protein structure characteristic of biologicals makes them more prone to induce an acute and/or chronic immune response. Biosimilars are a new class of drugs intended to offer comparable safety and efficacy to the reference, off-patent biological. They are not generic alternatives per se and are generally not interchangeable. Given their structural complexity, multifaceted manufacturing process and risk for immunogenicity, unique regulatory pathways are required for biosimilars. In this article, we review the clinical, safety and submission requirements for biosimilars in several major markets. We also highlight issues of ongoing debate amongst key stakeholders and examine some of the commercial challenges faced by developers of biosimilars. As the leader of biosimilars drug approval and product uptake, the EU is highlighted.