The millimetre and submillimetre spectral region (300 to 3000 Ilm or 1000 to 100 GHz) was until recently one of the few spectral regimes not fully opened up for astronomical studies. Thanks both to improvements in detectors and receivers and to the construction of large telescopes at high altitude sites this situation is improving very rapidly. Three major telescopes have been built recently and are coming into operation during 1987 and 1988, namely the 15m James Clerk Maxwell Telescope (JCMT) and the lOAm Caltech Submillimetre Observatory (CSO) telescope, both located on Mauna Kea, Hawaii, and the 15 m Swedish -ESO telescope (SEST) in Chile. Because a very wide range of astronomical problems can be tackled with these major new facilities there is a great deal of interest from the many potential new users anxious to become familiar with this rapidly developing field. During 1986 it became clear to British and Dutch astronomers involved in planning the commissioning and operation of the JCMT, that a summer school in this field would greatly benefit the potential and actual JCMT user community. With financial support from the SERC and supplemented by a grant from the ZWO, the Summer School on 'Millimetre and Submillimetre Astronomy' was held at Stirling University from June 21 to 27, 1987.

After a year's successful operation, the European DENIS project is now a scientific reality and its close cousin 2MASS (USA) is about to come into operation. The observational and data reduction processes of both DENIS and 2MASS are fully described in this volume. Already the impact of DENIS is making itself felt in the astronomical community in areas of research as diverse as cosmology, the evolution of galaxies, the interstellar medium, the search for brown dwarfs, and stellar structure and evolution. The first routine results from DENIS and the preliminary results from the 2MASS prototype camera are discussed and compared with other surveys across the wavelength spectrum, both space- and ground-based, including the Digitized Sky Survey, ISO and ROSAT.

Outstanding progress in near-infrared detection technology and in real-time image processing has led astronomers to start undertaking all-sky surveys in the 1--2 mum range (project DENIS in Europe and 2MASS in the U.S.A.), surveys which will have a considerable impact in various areas of astronomy. This book gathers the contributions of more than 80 specialists involved in fields of interest as different as low mass stars, late stages of stellar evolution, star formation, stellar populations of the Galaxy and the Magellanic Clouds, the local structure of the Universe, and observational cosmology. It describes the impact on these fields of the exhaustive data bases and catalogs of stars and galaxies that these surveys will provide. The considerable interest of these documents for the future of infrared space and ground-based projects and the complementarity with other currently ongoing or planned surveys in other spectral ranges are emphasized.

The previous Saas-Fee Advanced Course dedicated to the interstellar medium took place in 1972. The tremendous scientific advances that have occurred in this field since then, in particular owing to the availabihty of receivers working at completely unexplored wavelength bands, fuUy justified a new set of lectures. As a consequence, the members of the Swiss Society for Astrophysics and As tronomy voted that "The Galactic Interstellar Medium" should be the subject of the 1991 course. The 21st Saas Fee Advanced Course took place in Les Diablerets from 18 to 23 March 1991, gathering together about 80 participants from all over the world, but mostly from Europe. According to a rule that has proved to lead to success, but also to chal lenge the lecturers' energy, the format of a Saas-Fee Advanced Course consists traditionally of 28 lectures of 45 minutes which take place in the morning and late afternoon, leaving ample time for discussions, self-study, hiking or skiing. Despite the inordinate work load imposed, this year's lecturers felt that the subject was sufficiently dense to increase the lecture time by 1/3! This proved judicious and left more time for questions and discussions during the lectures.

Problems associated with a general scarcity of observations of the southern sky have persisted since the present era of galactic research began some sixty years ago. In his 1930 Halley Lecture A. S. Eddington commented on the observational support given to J. H. Oort's theory of galactic rotation by the stellar radial velocities measured by Plaskett o 0 and Pearce: " . . . out of 250 stars only 4 were between 193 and 343 0 galactic longitude [=GBP1: 225 < GBP11 < 15~; a stretch of one-third of the whole circuit was unrepresented by a single star. This is the operation which Kapteyn used to describe as "flying with one wing". By mathematical dexterity the required constants of rotation have been extracted from the lopsided data; but no mathematical dexterity can avert the possi bility that the neglected part of the sky may spring an unpleasant sur prise. As a spectator I watch the achievements of our monopterous avia tors with keen enthusiasm; but I confess to a feeling of nervousness when my turn comes to depend on this mode of progression. " During the past few years substantial gains have been made in securing fundamental data on the southern sky. Interpretations based on combined southern and northern surveys are producing a balanced descrip tion of galactic morphology. These matters were discussed at a Workshop held at the Leiden Observatory, August 4-6, 1982, attended by some 60 astronomers from 9 countries.

After a year's successful operation, the European DENIS project is now a scientific reality and its close cousin 2MASS (USA) is about to come into operation. The observational and data reduction processes of both DENIS and 2MASS are fully described in this volume. Already the impact of DENIS is making itself felt in the astronomical community in areas of research as diverse as cosmology, the evolution of galaxies, the interstellar medium, the search for brown dwarfs, and stellar structure and evolution. The first routine results from DENIS and the preliminary results from the 2MASS prototype camera are discussed and compared with other surveys across the wavelength spectrum, both space- and ground-based, including the Digitized Sky Survey, ISO and ROSAT.

Outstanding progress in near-infrared detection technology and in real-time image processing has led astronomers to start undertaking all-sky surveys in the 1--2 mum range (project DENIS in Europe and 2MASS in the U.S.A.), surveys which will have a considerable impact in various areas of astronomy. This book gathers the contributions of more than 80 specialists involved in fields of interest as different as low mass stars, late stages of stellar evolution, star formation, stellar populations of the Galaxy and the Magellanic Clouds, the local structure of the Universe, and observational cosmology. It describes the impact on these fields of the exhaustive data bases and catalogs of stars and galaxies that these surveys will provide. The considerable interest of these documents for the future of infrared space and ground-based projects and the complementarity with other currently ongoing or planned surveys in other spectral ranges are emphasized.

The millimetre and submillimetre spectral region (300 to 3000 Ilm or 1000 to 100 GHz) was until recently one of the few spectral regimes not fully opened up for astronomical studies. Thanks both to improvements in detectors and receivers and to the construction of large telescopes at high altitude sites this situation is improving very rapidly. Three major telescopes have been built recently and are coming into operation during 1987 and 1988, namely the 15m James Clerk Maxwell Telescope (JCMT) and the lOAm Caltech Submillimetre Observatory (CSO) telescope, both located on Mauna Kea, Hawaii, and the 15 m Swedish -ESO telescope (SEST) in Chile. Because a very wide range of astronomical problems can be tackled with these major new facilities there is a great deal of interest from the many potential new users anxious to become familiar with this rapidly developing field. During 1986 it became clear to British and Dutch astronomers involved in planning the commissioning and operation of the JCMT, that a summer school in this field would greatly benefit the potential and actual JCMT user community. With financial support from the SERC and supplemented by a grant from the ZWO, the Summer School on 'Millimetre and Submillimetre Astronomy' was held at Stirling University from June 21 to 27, 1987.

The International Astronomical Union has encouraged the study of our galaxy through a series of symposia. This volume contains the proceedings of IAU Symposium No. 84 on the Large-Scale Characteristics of the Galaxy, held in College Park, Maryland, from June 12 to 17, 1978. Symposium No. 84 was jointly sponsored by IAU 2ommission 33, Structure and Dynamics of the Galactic System, and by Commission 34, Interstellar Matter and Planetary Nebulae. The Scientific Organizing Committee consisted of F. J. Kerr (chairman), B. J. Bok, W. B. Burton, J. Einasto, K. C. Freeman, P. O. Lindblad, D. Lynden-Bell, R. Sancisi, S. E. Strom, H. van Woerden, and R. Wielen. The topics and speakers were chosen in order to emphasize current observational material and theoretical results pertaining to various morphological aspects of our galaxy. In preparing the program particular care was taken to relate recent work on other galaxies to the situation in our own galaxy. The meetings were held in the Center for Adult Education on the campus of the University of Maryland. The Local Organizing Committee consisted of B. M .. Zuckerman (chairman), A. P. Henderson, P. D. Jackson, T. A. Matthews, B. F. Perry, V. C. Rubin, P. R. Schwartz, F. W. Stecker, J. D. Trasco, and G. Westerhout. Joan Ball ably assisted this committee. The National Science Foundation made a financial con tribution to the general support of the meeting. The National Radio Astronomy Observatory assisted with some of the participants' travel expenses."

In June 1983 the Astronomical Institute of the State University of Groningen, founded by Kapteyn about 100 years ago, celebrated its one-hundredth anniversary. At the suggestion of its Chairman, R.J. Allen, the Kapteyn Institute invited the International Astronomical Union to mark the centenary by holding a Symposium on "The Milky Way Galaxy". The purpose of the Symposium was to review recent progress in the study of our Galaxy, to define current problems, and to explore prospects for future development. The Symposium programme would emphasize the large-scale characteristics of our Galaxy, and highlight both the historical development of our understanding of the Milky Way Galaxy and the importance of studies of external galaxies to this understanding. The Symposium was sponsored by four IAU Commissions: 33 (Structure and Dynamics of the Galactic System), 28 (Galaxies), 34 (Interstellar Matter) and 41 (History of Astronomy). The Scientific Organizing Committee, listed on page xviii, represented a broad range of nationalities and of expertise, including two historians of science. A meeting of the Committee, held during the IAU General Assembly at Patras, provided an excellent opportunity to discuss plan and format of the Symposium, topics and speakers; thereafter, the-Committee was regularly consulted by letter and telephone. IAU Symposium 106 was held at Groningen on 30 May - 3 June 1983, in the new building occupied by the Kapteyn Institute since January 1983. There were about 200 participants, coming from as many as 25 countries.