A very interesting application of metallic membranes is their integration in multifunctional reactors called membrane reactors. This integration allows carrying out reaction and separation in a single device, which results in higher yields and milder conditions compared with conventional reactor systems. These results translate in lower CAPEX (because of reduction of reaction volume and circumventing downstream separations) and reduction of OPEX (because of milder conditions used in the reactor). This chapter introduces the concept of membrane reactors and gives an overview of the latest results achieved in membrane reactor performance and operation.