This volume presents the first comprehensive review of globular cluster systems. Globular clusters are roughly spherical, densely packed groups of stars found around galaxies. Most formed at the same time as their host galaxies. They therefore provide a unique fossil record of the formation and early evolution of galaxies. This volume for graduate students and researchers summarises 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.