Research and innovation in multi rotor wind turbine systems (MRS) has made substantial progress over the past two decades. Research projects, such as INNWIND and X-Energy improved the design methods and knowledge on MRS, such as loads, structural design, operation, maintenance and more. The Vestas quad-rotor demonstrator revealed further insights, in particular on aerodynamic interactions and wakes. Numerous publications show the increasing interest in MRS and improved the knowledge in areas like blockage effects, wake recovery, control, loads and structural integrity. The installation and commissioning of the OceanX twin rotor as the first floating MRS, based on aerodyn's Nezzy2-concept gained a lot of interest in the wind energy community. Companies like Windcatching Systems and Myriad develop MRS with a substantially higher number of rotors. The presentation aims to describe the state of the art in research and development of MRS and dive into selected key factors such as loads, aerodynamics, structure, O&M and cost of energy. MRS-research gaps will be identified and outlined.