Globular clusters are roughly spherical, densely packed groups of stars found around galaxies. Most globular clusters probably formed at the same time as their host galaxies. They therefore provide a unique fossil record of the conditions during the formation and early evolution of galaxies. This volume presents a comprehensive review of globular cluster systems. It summarizes their observed properties, and shows how these constrain models of the structure of stars, the formation and evolution of galaxies and globular clusters, and the age of the Universe. For graduate students and researchers, this volume provides the definitive reference on globular cluster systems.