We have covered the exciting observational results, but without consideration of the experimental techniques or instruments required. An elementary understanding of mathematical physics and of quantum me chanics has been assumed, and a knowledge of basic astrophysics would be helpful. Older textbooks on interstellar physics have tended to develop the subject matter in an order which reflects the historical development of the field. Here a more pedagological approach has been adopted, based on our lecture course experience. We cover successively more complex physical systems found in the diffuse universe. Detailed mathematical rigour is eschewed in favour of provid ing the reader with a basic physical insight into these systems. Astrophysical problems are treated as practical applications of the physics. In practice, the material is generally ranked in order of decreasing entropy, since the hottest and most diffuse phases tend to be physically less complex.