Since their first detection 15 years ago, radio recombination lines from several elements have been observed in a wide variety of objects including HII regions, planetary nebulae, molecular clouds, the diffuse interstellar medium, and recently, other galaxies. The observations span almost the entire range from 0.1 to 100 GHz, and employ both single djsh and aperture synthesis techniques. The theory of radio recombination lines has also advanced strongly, to the point where it is perhaps one of the best-understood in astro physics. In a parallel development, it has become possible over the last decade to study these same highly-excited atoms in the laboratory; this work provides further confirmation of the theoretical framework. However there has been continuing controversy over the astrophysical interpre tation of radio recombination line observations, especially regarding the role of stimulated emission. A workshop was held in Ottawa on 24-25 August, 1979, bringing together many of the active scientists to review the field and discuss these questions of interpretation. A broad concensus has emerged: the subtleties of the line-formation process are understood, and the conditions under which reliable in formation can easily be extracted from the line measurements are known. It thus appears likely that the emphasis will shift increasingly from the study of the line phenomenon itself to further application in other areas of astrophysics, ranging from physical processes in plasmas (temperatures, densities, ionization structure), to the large-scale properties of our galaxy (abundances, kinematics, structure), and studies of extragalactic systems.

Astronomy and Astrophysics Abstracts, which has appeared in semi-annual volumes since 1969, is de voted to the recording, summarizing and indexing of astronomical publications throughout the world. It is prepared under the auspices of the International Astronomical Union (according to a resolution adopted at the 14th General Assembly in 1970). Astronomy and Astrophysics Abstracts aims to present a comprehensive documentation of literature in all fields of astronomy and astrophysics. Every effort will be made to ensure that the average time interval between the date of receipt of the original literature and publication of the abstracts will not exceed eight months. This time interval is near to that achieved by monthly abstracting journals, com pared to which our system of accumulating abstracts for about six months offers the advantage of greater convenience for the user. Volume 20 contains literature published in 1977 and received before February 20, 1978; some older literature which was received late and which is not recorded in earlier volumes is also included. We acknowledge with thanks contributions to this volume by Dr. J. BouSka, Prague, who surveyed journals and publications in Czech and supplied us with abstracts in English, and by Prof. P. Brosche, Bonn, who supplied us with literature concerning some border fields of astronomy.

Interstellar matter is one of the most active fields of research in present-day astronomy. Observational information spans the full electromagnetic spectrum from gamma rays through rocket-ultraviolet, optical, infrared and millimeter to long radio waves. Results of research in physical chemistry find as much application as mathematical methods. Interstellar matter plays a leading role in studies of our Galaxy and of external galaxies, and contributes increasingly to stellar astronomy. At the 16th General Assembly of the International Astronomical Union, held in August 1976 in Grenoble, France, the many new developments in this vast field were surveyed in a number of sessions of Commission 34 (Interstellar Matter), mostly jointly with other Commissions of the Union. Separate sessions were devoted to: The hot interstellar gas phase, Interaction of stars and interstellar medium, Interstellar molecules and dust, The large-scale distribution of interstellar matter in the Galaxy, and Interstellar matter in external galaxies. Twenty-four invited review papers were presented and discussed in these sessions. The quality and success of these topical reviews made it seem desirable to make them available to a wider audience. Professor Edith Muller, the new General Secretary of the IAU, enthusiastically supported the idea. Most importantly, the reviewers - who had originally been pro mised that an oral paper was the only requirement - agreed to prepare written versions. I am grateful to Mrs. Muller, to the authors, and to Reidel Publishing for their collaboration in the preparation of this book.