In recent years it has become apparent that con­ tributions to our knowledge about the interstellar medium are made by practically all forms of astronomy ranging from radio- to gamma ray observations, and from cosmic ray measurements. It was thus thought fruitful to arrange for an interdisciplinary meeting of astron­ omers from the various fields of investigation, and of high energy astrophysicists. This meeting took place at Schliersee (Bavaria) from April 2 to 13, 1973. Lectures and some of the discussions held during that Advanced Study Institute are reproduced here. unfortunately, no manuscripts of the two lectures about infrared and cosmic ray observations were re­ ceived and these are thus not available here. However, it was thought to be more important to proceed now with the publication. The organisers are very grateful to Miss H. Eichele for her technical help during the meeting. The or­ ganisation of the Institute and the publication of the proceedings would have been impossible without the very great efforts and help of Mrs. M. Brunner and Mrs. D.Preis. I would also like to express my gratitude to my col­ leagues K.W. Michel and S. Drapatz for their many con­ tributions towards the success of the meeting.