The combination of reaction and separation in a system in the form of a membrane reactor represents a process intensification, through which various advantages can be generated. For example, a gentle retention of homogeneous transition metal catalysts paired with comparatively low-energy consumption can be realized. Depending on the operating principle, membrane reactors can be divided into extractors, contactors, and distributors. Extractors allow a selective product separation from a reaction mixture, contactors offer two immiscible phases a defined phase interface for mutual mass exchange within a porous membrane, and distributors enable a targeted dosing of substrates. Within this contribution, both basics for the application of membranes and application examples of membrane reactors are shown.