This volume contains the fifteenth tri-annual reports of the Presidents of the forty Commissions of the International Astronomical Union; it refers to the progress in our discipline during the three years 1970, 1971 and 1972. As compared to earlier volumes a gradual change in character is unmistakable. The ever increasing flow of publications, combined with the obvious necessity to keep the Reports at a reasonable size and price level has gradually forced the Commission Presidents to be more selective than before in drafting their Reports. I have certainly stimulated them into that direction - in order that Reports like these be valuable and lasting, it seems imperative that the individual contributions have the character of a critical overall review, where a fairly complete summary is given of the major develop ments and discoveries of the past three years, and in which the broad developments and new trends be clearly outlined, while at the same time essential problems for future research are identified. With respect to the latter item I have suggested the Commission Presidents to add to their reports a brief section on scientific priorities for future research in the field of their Commissions. In order to save space I have suggested to Commission Presidents that references to published papers are given on the basis of their number in the published issues of Astronomy and Astrophysics Abstracts. For instance, the indication (06. 078. 019) or (AAA 06. 078.

IAU Transactions are published as a volume corresponding to each General Assembly. Volume A is produced prior to the Assembly and contains Reports on Astronomy, prepared by each Commission President. The intention is to summarize the astronomical results that have affected the work of the Commission since the production of the previous Reports up to a time which is about one year prior to the General Assembly. Volume B is produced after the Assembly and contains accounts of Commission Meetings which were held, together with other material. The reports included in the present volume range from outline summaries to lengthy compilations and references.

The International Astronomical Union (IAU) was founded in 1919, in the wake of the First World War, together with its sister Unions in related natural sciences. It will thus turn 100 years in 2019. Written by a mixed team of insiders and outsiders, this book presents the IAU in the changing context of the historical, scientific and technological development of astronomy during the past 100 years. While much important scientific progress took place already before 1945, the book naturally focuses on the accelerating evolution during the second half of the century. In the past few decades, the previously narrow IAU focus on organising professional astronomy has broadened to include societally relevant activities such as addressing the hazard of asteroid impacts, the planetary status of Pluto in the Solar System, and the hugely successful International Year of Astronomy. Most recently, it is spearheading a combination of science literacy and public outreach. The book will be of interest to professional astronomers as well as an astronomically interested general audience. The book features live personal interviews with as many of the key actors as still possible.

The General Assemblies of the International Astronomical Union are landmarks in the life of the world-wide astronomical community, as they review, at triennial intervals, the progress made in this scientific field, promulgate the most spect acular astronomical achievements, formulate scientific programmes for the years to come and, last but not least, deal with the administration and finances of the IAU. The Reports on Astronomy 1976, published as Transactions XVIA (in 3 volumes) before the XVlth General Assembly, are a synopsis of the work done in astronomy from 1973 to 1975. The volume "Highlights of Astronomy , as presented at the XVlth General As sembly of the IAU in Grenoble, 1976" includes some selected scientific topics, and will appear in the first half of 1977. Apart from the Invited Discourses and the Proceedings of the seven Joint Discussions, the Highlights volume No.4 con tairsthe proceedings of two Joint Commissions Meetings.