This volume contains the written version of the lectures delivered at the 21st course in the series of renowned Saas Fee Advanced Courses of the Swiss Society for Astrophysics and Astronomy. The topics covered by the three leading experts are as follows: "Distribution and Observational Properties of the ISM" by Burton, "Large Scale Dynamics of the ISM" by Bruce Elmegreen, and "Physics and Chemistry of Molecular Clouds" by Reinhard Genzel, Although a complete coverage of the interstellar medium is nowadays quite impossible in a one-week course, these written lectures give one of the most up-to-date presentations of the essential facts about the interstellar medium. Over the coming years this volume should remain invaluable to students and active or new researchers in the field.