This book contains the proceedings of a workshop held in Schlos Ringberg to assess developments in molecular cloud research over the last 25 years, and to discuss trends for future research in the field of molecular line astronomy. The topics include the morphology, formation, and lifetimes of molecular clouds, and their relation to star formation. Also, the chemical and isotopic content of these clouds is reviewed, and comparisons with molecular clouds in external galaxies are made. This rather complete survey of this important field of research addresses researchers in astronomy and students alike.