Arriving at the next step outwards, in this chapter we look at clusters of galaxies. The first question asked is how we define a cluster of galaxies. A simple answer would be that a cluster is a set of galaxies bound together by its own gravity, and not expanding internally as the universe expands. But the smaller sets of galaxies are called groups not clusters. Here again it does not make sense to adopt a definition. Better to describe them. We start with a map A of the clusters and groups within 100 million light years of the Milky Way, and from this we pick out our own local cluster, the Virgo Cluster, to describe its overall structure, and the distribution of its spiral and elliptical galaxies. We then see how measuring the velocity of recession of galaxies supposedly within a given cluster helps to identify its members. Next come detailed descriptions of some of its outstanding member galaxies, featuring the giant elliptical M87 with its huge supermassive central black hole, with its long high energy opposed jets. Another archetype galaxy in Virgo is M100 which we show as an example of how the Hubble Space Telescope’s initially blurred images were cured by later attention by astronauts on the space Shuttle. This is followed by a quick look at some dwarf galaxies in Virgo. A second important nearby cluster, the Coma cluster is described, showing large scale maps in visible light and X-rays, and optical images of some of its more important galaxies. The most important catalogue of galaxy clusters, by George Abell, is then described, and the chapter ends with a detailed look at some Abell clusters as imaged by the Hubble Space Telescope in the visible and near infrared, as well as with NASA’s Chandra X-ray satellite observatory.