There are two questions that we can ask ourselves in order to describe this workshop. The first question is a double question: why a conference on this subject and why a workshop? The first idea of organizing this workshop came while reading the scientific objectives of one of the instruments onboard the ISO satel lite (a phase A document concerning the IR camera). On going through the scientific motivations for building the instrument I realized with surprise that no mention was made of Planetary Nebulae (PN). At present this is no longer true. There is a chapter indicating the capabilities of the camera in the PN field and what we can reasonably expect from that instrument. But it was at this moment that the first idea of organizing a workshop on the subject of PN came. Of course there are other, stronger motivations. The first one is that I think this is the right moment after IRAS. I think we all spent the last two or three years working on IRAS data. IRAS represented a corner-stone for those working on Planetary Nebulae: the amount of data that came out of the instruments onboard the satellite was enormous and opened up new ways of looking at planetary nebulae, as well as at other fields.

The purpose of this book is to give a detailed description of the planetary nebulae including the relevant astronomical observations and their interpretation. Considerable attention is given to the evolution of these objects as well as to their physical characteristics. I t is hoped that the book be useful to both advanced research workers and to students with some background in astronomy. In this regard, the book should serve as a text as well as a reference work. The many tables included are expected to be useful for both purposes. The references are generally not included in the text except for historical purposes in an effort to improve readability. References are given at the end of each chapter together with sufficient text to describe their content. No attempt has been made to make the list of references complete; on the contrary it has generally been limited to the most recent literature on the subject which in turn refers to earlier research. Again, exceptions have been made for references of historical interest.

Planetary nebulae are a keystone for the understanding of the evolution of stars, for deep insights into the physical processes prevailing in highly excited dilute nebulae, and for the chemical evolution in galaxies. These objects, displaying an intriguing morphology, have a `short' lifetime of a few tens of thousands of years, and have become one of the best studied classes of celestial sources. However, despite large and successful efforts from both the observational and theoretical side, planetary nebulae still keep some of their secrets (like the widely unknown distances) and will undoubtedly also be objects of thorough investigations in the years to come.

Planetary nebulae present a fascinating range of shapes and morphologies. They are ideal laboratories for the study of different astrophysical processes: atomic physics, radiative transfer, stellar winds, shocks, wind-wind interaction, and the interaction between stellar winds and the interstellar medium. In addition, planetary nebulae provide information about the late stages of stellar evolution. In the last five years studies of planetary nebulae have progressed very rapidly and new phenomena and insights have been gained. This is partly due to new observations (e.g. from the Hubble Space Telescope, the ISO satellite and new infrared and millimeter spectrographs) and partly to the advancement of hydrodynamic simulations of the structures of planetary nebulae (PN). Many of these new results were reported at IAU Symposium 180 in Groningen, the Netherlands, on August 26 to 30, 1996. This symposium was dedicated to one of the pioneers of PN research: Stuart Pottasch. These proceedings contain chapters on: Introduction to PN with the basic parameters Distances of PN The central stars of PN The envelopes of PN The evolution from AGB to PN The evolution from PN to white dwarfs PN in the galactic context PN in extragalactic systems The future of PN research . The book contains 29 reviews and more than 200 shorter contributions.

This written account of the Symposium on Planetary Nebulae was prepared from manuscripts submitted by the participants. Nearly every paper that was presented at the meeting is reproduced here, in either complete or abbreviated form. The dis cussions have been somewhat shortened and rearranged, but we have tried to preserve the essential points and the general tenor of the exchanges. Participants who spoke in the discussion were asked immediately for written remarks, which were then edited, reproduced, and circulated at the meeting by the highly effective local Secretariat organized by Dr Perek. In addition, notes of the discussion taken by Mrs Edith F. Swan and by the undersigned were used. We wish to thank all the authors for their unusually good cooperation. We are especially grateful to Dr Minkowski, who kindly provided many excellent repro ductions of Mount Wilson and Palomar photographs, mostly taken by himself, of various planetary nebulae. We are particularly indebted to Mrs Swan, who attended the Symposium, made notes on the papers and discussions as they occurred, and did much of the checking and editing of the manuscripts. In addition, we are very grateful to Mrs Evelyn Seaver, who also did much of the checking, editing, and retyping of manuscripts, and to Dr B.L. Webster, Miss Rebecca Todd, Mr Joseph Tapscott, and Mr Dennis Schatz, who provided excellent assistance in the preparation of this volume.

Recently, improved observational capabilities have allowed the study of fainter and fainter extra-galactic planetary nebulae in galaxies well beyond the Milky Way. This book result from a workshop held at ESO headquarters in Garching in 2004, the first devoted to Extra-galactic Planetary Nebulae. A wide range of topics is covered, from stellar and nebular astrophysics to galactic dynamics and galaxy clusters, making this a reference of broad astrophysical interest.

This book provides a review of the work in progress on the infrared spectral region of stars. The most important previous mission, the Infrared Astronomy Satellite (IRAS) was launched in 1983. Although it was only operating for one year, in that time it surveyed the whole sky for infrared emitters of many different kinds providing a fundamental catalogue of thousands of objects. As a consequence of its success, a new mission to observe the infrared sky is being planned. The Infrared Space Observatory (ISO) is scheduled for a launch in May 1993. It should add substantially to our knowledge of the stars. The volume is based on the proceedings of an international conference held in Montpellier, France. It provided an excellent forum for the experts in the field to get together and evaluate the status of current knowledge and analyse the instrumentation.