Multi-rotor wind turbines promise lighter structures and higher capacity, yet operating experience remains scarce. This talk reviews rotor–rotor interaction across three scales—blade-to-blade, rotor-to-rotor and array flow—drawing on lifting-line vortex, actuator-line RANS (Reynolds-averaged Navier–Stokes)/LES (Large eddy simulation) and actuator-disk CFD (Computational fluid dynamics) studies. Focus is on aerodynamic interactions shaping performance and load distributions, with brief reference to implications for structures, fatigue and control. Consistent findings, open questions, and future directions for aerodynamic research on multi-rotor arrays are outlined.