The question: "What are clusters of galaxies made of?" would have received the most obvious answer until 1933, when the main cluster component, dark matter, was discovered. About 40 years later, another component of galaxy clusters, the hot, X-ray emitting, intra-cluster gas, was discovered. Today, large spectroscopic data-sets of cluster galaxies are available, as well as X-ray data for the intra-cluster gas of many clusters. Together, these data are used to determine the relative distributions of the different cluster components, galaxies, intra-cluster gas, and dark matter, and their relative mass fractions as a function of clustercentric distance. These informations constrain cosmological models for the formation of structures, the nature of dark matter, and the evolutionary history of cluster galaxies of different types.