Explore the intricacies of driven open quantum systems in this comprehensive lecture by Sebastian DIEHL from the Institute for Theoretical Physics at the University of Cologne, Germany. Delve into topics such as the Lindblad equation, probability conservation, deterministic limits, and Fourier expansion. Examine the classical action principle and its relation to dissipation. Investigate classical field theory, source terms, and their applications to polaritons and condensation. Gain insights into scaling phenomena and the transition from micro- to macrophysics in open quantum systems. This in-depth presentation offers a thorough exploration of the subject matter, suitable for advanced students and researchers in the field of condensed matter and statistical physics.