A biosimilar is a biological medicine that is very similar to another one already marketed in the European Union (EU) whose patent has expired and the differences between which are not clinically significant, in other words do not affect clinical practice. More biosimilars are slowly being developed and, consequently, the experience with their use is increasing. The European Medicines Agency (EMA) considers the biosimilars approved in the EU to be interchangeable, and has signed a statement to reduce the uncertainty regarding interchangeability of biosimilars in clinical practice. The aim of this bulletin is to define the basic concepts, describe the comparability, authorization and interchangeability processes, evaluate the scientific evidence regarding the efficacy, safety and economic impact of the switch, and to describe the current situation of biosimilars in Spain.