Associations of galaxies range from pairs and small groups, through giant clusters containing over a thousand galaxies, to the vast structures on scales much greater than clusters such as the vast ‘walls’ and voids observed in the distribution of galaxies. Clustering occurs on all scales and very few galaxies can be considered truly isolated. Rich clusters of galaxies are of particular interest because they are the largest gravitationally bound systems in the Universe. The gravitational potential of the cluster is defined by the distribution of dark matter, the mass of which greatly exceeds that of the baryonic matter, such as that contained in the stars in galaxies and the associated interstellar gas and the intracluster gas. The deep gravitational potential wells of clusters can be observed directly through the bremsstrahlung X-ray emission of hot intracluster gas which forms a hydrostatic atmosphere within the cluster. The hot gas can also be detected through the decrements which it causes in the Cosmic Microwave Background Radiation as a result of the Sunyaev–Zeldovich effect. Gravitational lensing has proved to be a very powerful tool for defining the large scale distribution of dark matter in clusters, as well as in individual galaxies within them. Interactions of galaxies with each other and with the intergalactic medium in the cluster can be studied and radio source events can strongly perturb the distribution of hot gas.