The crystal structure of twenty structurally different types of high-temperature oxide superconductors is discussed. This is followed by a detailed review of the phase relationships and the growth of single crystals belonging to the following systems: La–Sr–Cu–O, Ba–Y–Cu–O, Bi–Sr–Ca–Cu–O, Nd–Ce–Cu–O, Tl–Ba–Ca–Cu–O, Ba–K–Bi–O, and Ba–Rb–Bi–O.