Clusters of galaxies are the largest virialized structures in the Universe. Though they were originally defined as large agglomerations of galaxies on the sky, later it was found that their substance consists of a large amount of dark matter and hot gas occupying the gravitational potential. Because of this, clusters have been among the most important objects to examine the existence and properties of dark matter. Their strong gravitational field causes gravitational lensing, which is useful for studies on cosmology and galaxy evolution. High-energy electrons in the hot gas of clusters scatter the photons of the Cosmic Microwave Background (CMB) via inverse Compton scattering, referred to as the Sunyaev–Zel’dovich (SZ) effect. The SZ effect also provides a valuable tool for cosmology. Clusters have evolved by the merging of smaller dark halos and are relatively young, having mainly formed at \(z < 1\) . They not only evolve as a whole, but their member galaxies undergo rapid evolution because of their size and mass. This is a typical example of the environmental effect on galaxy evolution. The intracluster medium (ICM) contains a significant amount of heavy elements. These elements must have been produced by galaxies, but the mechanism by which they are transported to the ICM remains an exciting topic to explore.