Stellar populations, the building blocks of galaxies, are direct tracers of a galaxy's star formation history, its chemical enrichment and the assembly of galaxies in the Universe. This last decade has witnessed a revolution in our observations of galaxies; with larger telescopes and new instruments allowing us to look deeper in the Universe, and to study nearby galaxies in greater detail. IAU Symposium 241 reports the considerable progress made in recent years in this topic. Theorists, observers, and researchers of resolved and unresolved stellar populations discuss the ingredients of stellar population models and compare them to new data, forcing theorists to develop more refined models and methods to derive the physical parameters of stellar populations. New results from the Milky Way, the Local Group, and nearby and distant galaxies are presented.

The concept of Stellar Populations has played a fundamental role in astronomy in the last few decades. It was introduced by Walter Baade after he was able to resolve the Andromeda Nebula and its companions into stars when he used red-sensitive plates and realised that there were two fundamentally different Herzsprung-Russell diagrams in our and these nearby galaxies (common stars in the solar neighborhood versus globular clusters). This result was published in two papers in 1944 in volume 100 of the Astrophysical Journal. Subsequent research gave the concept a much firmer basis and at the famous Vatican Symposium of 1957 resulted in a general scheme of the concept and a working hypothesis for idea's on the formation and evolution of the Galaxy. This has been a guiding principle of studies of our and other galaxies for decades. Some years ago it seemed to us appropriate to commemorate Baade's seminal work in 1994, when it would have its 50-th anniversary, and to review its present status and also its role in contempory understanding. While we were in Leiden for an administrative committee, we discussed the matter again and over beers on October 29, 1991 we decided the take the initiative for an IAU Symposium on the subject during the 1994 IAU General Assembly in Den Haag, the Netherlands.

IAU Symposium 262 presents reviews on the current understanding of the theories of stellar evolution, galaxy formation and galaxy evolution. It emphasises what we have learned in the past few years from massive surveys covering large portions of the sky (e.g. SDSS, HDF, UDF, GOODS, COSMOS). Several critical aspects of research on stellar populations deserve further effort in order to be brought in tune with other areas of astrophysical research. The next ten years will see the opening of major observatories that will increase the quality and quantity of astronomical data by orders of magnitude. The expected benefits from these instruments for the study of stellar populations are explored. This critical review of state of the art observational and theoretical work will appeal to all those working on stellar populations, from distant galaxies to local resolved galaxies and galactic star clusters.

The concept of Stellar Populations has played a fundamental role in astronomy in the last few decades. It was introduced by Walter Baade after he was able to resolve the Andromeda Nebula and its companions into stars when he used red-sensitive plates and realised that there were two fundamentally different Herzsprung-Russell diagrams in our and these nearby galaxies (common stars in the solar neighborhood versus globular clusters). This result was published in two papers in 1944 in volume 100 of the Astrophysical Journal. Subsequent research gave the concept a much firmer basis and at the famous Vatican Symposium of 1957 resulted in a general scheme of the concept and a working hypothesis for idea's on the formation and evolution of the Galaxy. This has been a guiding principle of studies of our and other galaxies for decades. Some years ago it seemed to us appropriate to commemorate Baade's seminal work in 1994, when it would have its 50-th anniversary, and to review its present status and also its role in contempory understanding. While we were in Leiden for an administrative committee, we discussed the matter again and over beers on October 29, 1991 we decided the take the initiative for an IAU Symposium on the subject during the 1994 IAU General Assembly in Den Haag, the Netherlands.

Star clusters are important to many areas of astronomy, and as the basic building blocks of galaxies, they can be used as key diagnostic tools within a wide range of disciplines in astrophysics. Star cluster populations are powerful tracers of the formation, assembly and evolutionary history of their parent galaxies. Although their importance has been recognised for decades, only in recent years has this area seen a major investment in time and effort. IAU Symposium 266 consolidates the expertise of leading researchers from a variety of topical subfields in astrophysics, to provide a comprehensive presentation of cutting-edge developments in theory, observations and simulations of star clusters and star cluster systems across a range of sizes and epochs. This volume gives an account of this forefront research, answering fundamental questions that will improve our understanding of numerous related issues and show how this field will take its next major step forward.