This second volume of Reviews in Modern Astronomy continues the new series of publications of the Astronomische Gesellschaft (AG). In order to bring the scientific events of the meetings of the society to the attention of the worldwide astronomical community, it was decided to devote a new annual publication, Reviews in Modern Astronomy, exclusively to the invited reviews, the Karl Schwarzschild lectures, and the highlight contributions from leading scientists reporting on recent progress and scientific achievements at their institutes. Volume 2 comprises the contributions presented during the spring meet­ ing of the AG at Friedrichshafen in April 1989, which was dedicated to the topic "Astrophysics with Modern Technology - Space-based and Ground-based Systems", as