This book will introduce advanced concepts and topics of solid‎-state theory‎.‎ To this end we need a tool box that enables us to treat electron‎-electron interactions‎,‎ and possibly also electron‎-phonon or phonon‎-phonon interactions in some well‎-defined‎,‎ approximate way‎.‎ The most advanced‎,‎ yet still feasible approach‎,‎ is the Green‎'‎s function technique and the perturbation expansion using Feynman diagrams‎.‎ This technique allows the interacting system to be treated as ‎'‎solid‎'‎ within well‎-characterizable approximations‎,‎ still widely used today‎,‎ but also provides the language used in modern computer‎-based simulations as well as for the interpretation of experiments‎.‎