The GW-method is a Green’s function based approach from the field of many body perturbation theory. It s most commonly used to perturbatively correct the electronic structure obtained from, for instance, density functional theory calculations. Formally derived already fifty years ago the first applications date back thirty years and only this decennium the method has become a standard approach for solid-state applications. The application to molecular systems is however still in an early stage. In the first part of this seminar the basic concepts of the GW method are introduced and the general problems are presented. In the second part the specific techniques and approached for the application to molecular systems are discussed. Finally we come back to solids and discuss the automatization of GW with the aim of high throughput computation.