Biosimilars are follow-up versions of innovative medicines with proved high similarity to the reference product. There may be some minor differences in clinically inactive components of a molecule which can be clinically significant, but they do not substantially affect the high resemblance to the original drug in their safety and potency. Introduction of the biosimilars brings hope to reduce the cost of treatment, so their availability will increase further as the expiration of patents on biotechnologically produced drugs will follow, which is currently taking place. There are new biosimilar drugs being introduced recently; these are monoclonal antibodies. They are much more structurally complicated compared to the first biotech drugs such as insulin, and their production processes are much more complex. Biosimilars as the end products of such sophisticated technology are affected by many variables. It is not possible to create "biogenerics," but only biosimilar drugs. Therefore, despite their similarities to the reference molecules, biosimilars are too different to forego clinical trials. Clinical trials of biosimilars need to be much broader than for generics, but they are limited compared to the reference biologicals' testing. Placebo controlled clinical trials are not required before registering a new biosimilar. Immunogenicity is a major source of concern related to biosimilars, although there may be the same problem with the original drugs.