in this volume three extended lectures addressing both students and experienced astronomers and astrophysicists cover a wealth of material on interstellar matter. the first lecture details the most recent observationaldata on interstellar matter inside and outside our galaxy, such as the global distribution of matter and atomic or molecular gas and its kinematics. the second treats its large-scale dynamics, including instabilities and star formation. in the third the physics and chemistry of molecular clouds are discussed. this book can be used as a textbook for advanced students in astrophysics.