What do companies in the SME sector expect from Industry 4.0? Where do they hope for advantages, where are there tangible and concrete benefits? These questions are difficult to answer without first investing large sums in equipment and personnel. This is because existing plants, i.e. machines in which investments were made in the past and which are still performing their service perfectly, often do not provide any data for control and condition monitoring. With the so-called "I4.0 retrofit", these machines can be upgraded for Industrie 4.0 applications in order to be operated more safely, efficiently, flexibly and transparently. To this end, Fraunhofer IOSB-INA and the VDMA have for the first time written a guide that provides important assistance, especially for small and medium-sized enterprises, in the digitization of their existing machines.