Nowadays, medium to large sized fans in traditional applications are increasingly replaced by fanarrays, that is, a number of usually smaller fans operating in parallel. Although the aerodynamic efficiency of single fans still rises through further development, the research and development the field of fan systems will get even more important in the future. Besides redundancy, these fan arrays offer additional degrees of freedom. In applications with varying operating points, high improvement in system efficiency and high energy savings, respectively, can be achieved by selectively turning off some of the fans in an array and controlling the operation of the remaining active fans accordingly. We propose a method for the optimal control of such fan arrays. In our approach, the arrays can be composed of an arbitrarily number of differed sized fans and/or fans with different fan curve characteristics. There are only two prerequisites present in the theoretical derivation of the method. It is assumed that the speed of each fan in the array of fans can be variably controlled up to its maximum rotational speed. Moreover, it is assumed that no backflow occurs in situations where fans are turned off. In reality, this can be achieved using, i.e., some kind of shutters or flaps. For a given fan array and operating point, the method yields the optimal number and combination of active fans and their optimal rotational speed, which maximizes the system efficiency. Lastly, applying this method to the inverse problem, where the shape of the actual array is yet unknown yields an optimal design of fan arrays for given device and fan size constraints.