Welcome to the web page of Theory of Quantum systems and Complex systems (TQC). We study systems for which quantum mechanical behaviour becomes macroscopic, e.g. superconductors, liquid helium, atomic superfluids, polariton superfluids, ... We try to understand and predict their behaviour, with Feynman's path integral theory as our tool of choice. Our study and refinement of this theory has also led to several "spin-off" research topics, e. g. econophysics. Furthermore, we try to transfer our knowledge to students in several bachelor and master courses, and to the general public in outreach activities.