The ZHAW Institute of Public Management has been involved with operational numbers of libraries for several years. Next to the yearly libraries benchmarking, the institute does research on controlling tools, or performance measurement systems respectively. Operational numbers are rarely interpreted according to their duration of importance, meaning if they are only applicable in the short run or sustainably in the long run. Additionally, performance orientation with the goal of increasing efficiency, is rather one-sided and does not meet the increasing demands from society, and therefore the donors of public organizations, of more effectiveness. A change of perspective towards more impact orientation can be observed in the public sector, which is likely to increase in the future. An evidence-based controlling tool for libraries, based on applicable operational numbers, is therefore of the essence. Within the tool, the particularities of different library types (scientific libraries, city and regional libraries) are to be considered. The controlling tool is developed, introduced and evaluated in cooperation with libraries.