Biosimilars, based on biopharmaceuticals approved by regulatory agencies that are no longer under patent protection, have efficacy and safety comparable to their reference products, and are a new therapeutic option to treat inflammatory diseases. Biosimilars must be distinguished from 'biomimics' or 'biocopies', which are marketed in some countries but have not been evaluated according to the stringent regulatory pathway used for biosimilars. CT-P13, based on infliximab, was the first biosimilar approved for the treatment of inflammatory diseases; however, some countries did not allow extrapolation of indications to all eight diseases for which the reference drug infliximab is approved. Antidrug antibodies can reduce drug levels and affect clinical efficacy, but although available data suggest that biosimilars and their reference products have comparable immunogenicity, this important property might differ between individual biopharmaceuticals. This Review discusses biosimilars already approved within the past 3 years to treat rheumatic diseases, as well as others that are currently under development. The main challenges posed by biosimilars are also addressed, such as the extrapolation of indications to diseases only studied for the reference drug, and the definition of strategies for adequate pharmacovigilance to monitor biosimilars after marketing approval.