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.

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.

Every 5 years since 1967 a meeting has been held to discuss the subject of planetary nebulae and their central stars. Previous meetings have been held in Tatranska Lomnica (Czechoslovakia); Liege (Belgium); Ithaca, New York (U. S. A); and London (Great Britain). IAU Symposium 131 was sponsored by IAU Commision 34, on Interstellar Matter and co-sponsored by IAU Commisions 35 and 36 on Stellar Constitution and Theory of Stellar Atmospheres. The symposium was held at the Universidad Nacional Autonoma de Mexico in Mexico City, October 5-9, 1987. It took place in one of the old buildings of the University of Mexico in the downtown area. The inner patio of the building provided very pleasant surroundings for the poster sessions and for extensive discussions among the participants. The meeting was attended by 160 scientists from 22 countries. The Scientific Organizing Committee,under the chairmanship of J.B. Kaler, prepared a comprehensive scientific program based on a set of invited presentations. All contributed papers were presented in poster form. The Scientific Organizing Committee would like to thank the staff of the University of Illinois Department of Astronomy: Dr. Ron Allen for granting financial support; Carol Stickrod, Louise Browning, Deana Griffin and Sandie Osterbur for their help with the organization. IAU provided economic assistance to a group of young astronomers.

The term proto-planetary nebulae (PPNe) in the context of the late stages of stellar evolution was created only slightly more than 20 years ago to express the belief that in the near future these objects will become planetary nebulae (PNe). The first proto-planetary nebulae (called also post-Asymptotic Giant Branch, or shortly post-AGB objects) AFGL 2688 and AFGL 618 were discovered in mid seventies in course of the Air Force Sky Survey. Investigation of this phase of stellar evolution developed very rapidly in 1980's after the IRAS mission when it became clear that proto-planetary nebulae emit a significant part of their energy in the mid-and far-infrared. Hundreds of new candidates have been proposed but the recognition of the real proto-planetary nebulae is not a simple task and needs a substantial effort to exclude cases that represent different evolutionary stages. High resolution spectroscopy of stellar atmospheres is of much importance in this respect. Surprisingly, only a small group of central stars, the so called 21 11m emitters, show chemical signatures of the 3 rd dredge up process. Very recently, a more detailed studies of mid-infrared spectra from the Infrared Space Observatory (ISO) allow for a better understanding of chemical composition and evolution of circumstellar material around these stars. A new impetus in the field of proto-planetary nebulae research was started in the 1990's with high spatial resolution imaging in mid-infrared and optical wavelength ranges.

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.

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.