This introductory chapter discusses the observations on which our understanding of globular clusters lies. Successive sections deal with photometry, chemical abundances, the details of color-magnitude diagrams, the distance scale, luminosity and mass functions, and the lower end of the main sequence. An appendix treats the dynamical role of binaries in globular clusters.

Astronomy aims at an understanding of the facts and phenomena that we see, and the processes by which they came about—and in the best of possible cases, the recognition of why they had to be this way and could not have been otherwise. The first stage in this endeavor is to see what is there, and, to the extent that we can, how it became that way.

Globular clusters can in many ways be considered the crossroads of astronomy. They have played a central role in the unfolding of our astronomical understanding, to which they bring two singular advantages: first, each cluster (with a possible rare exception) is a single and specific stellar population, stars born at the same time, in the same place, out of the same material, and differing only in the rate at which each star has evolved. Such a group is much easier to study than the hodgepodge that makes up the field stars of the Milky Way. Second, globular clusters are made up of nearly the oldest—perhaps the very oldest—stars of the Universe, and as such they give us an unparalleled opportunity to probe the depths of time that are the remotest to reach.