The introduction of biologic drugs represents the most significant advance in the management of immune-mediated inflammatory diseases for a decade. However, complex proteins are expensive to produce and manufacture. Biosimilar versions of established biologics are becoming available as another version of the reference medicinal product and are expected to provide substantial cost savings. However, because of their complexity, the approval of biosimilars requires strict controls to ensure that all therapeutically relevant characteristics are comparable to the reference medicinal product. This review summarizes the scientific principles and data requirements underpinning regulatory approval of biosimilars and the assumptions that enable extrapolation of data between indications. These important concepts are exemplified by CT-P13 (Remsima®, Inflectra®), the first biosimilar monoclonal antibody approved in Europe.