Book excerpt: 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.