When, upon my return to Holland, I started to teach an advanced course on the interstellar medium in 1998, I quickly realized that there was no suitable textbook available. There is, of course, the incomparable monograph by Spitzer, Physics of the Interstellar Medium (1978, New York: Wiley and Sons). But that book is quite challenging and not very suitable for a student course. Moreover, by now, it is very dated. Over the intervening years, our insights into the basic physics of the interstellar medium have much improved thanks, for example, to the opening up of the infrared and submillimeter windows. In particular, molecules, which we now know to be deeply interwoven into the fabric of the Universe, play only a little role in Spitzer's book. When Eddington made his famous remark, “Atoms are physics but molecules are chemistry,” he merely expressed, on the one hand, the dream of a physicist of a simple universe, which can be caught in a single equation, and, on the other hand, the dread of a reality where solutions are never clean and simple. The latter is of course obvious to a chemist and it is now abundantly clear that Eddington's fear has turned into reality, even for astronomy. Present-day graduate students will require an intimate knowledge of molecular astrophysics in order to be active in the field of the interstellar medium of our own or other galaxies whether it is in the here and now or all the way back in the early Universe.