The investigation of frustrated spin systems plays a central role in todays condensed matter physics. In particular, the search for exotic non-magnetic spin-liquid phases has become a longstanding problem in theoretical as well as in experimental physics. A detailed understanding of this type of matter could trigger new developments in the young field of quantum computation, and is also essential for the theory of high temperature superconductivity. However, there are not many numerical methods available which are able to deal with models for frustrated spin systems. Johannes Reuther develops new analytical and numerical methods for calculating groundstate properties of a large class of spin models on the basis of the pseudofermion representation of spin operators. In order to perform infinite resummations of perturbation theory in the couplings, the functional renormalization group (FRG) method is applied. This way, the author determines the quantum phase diagrams of Heisenberg spin systems on different two-dimensional lattices and characterizes the non-magnetic states found therein.