ABSTRACTSince the discovery of the high-Tc superconductor in the La-Ba-Cu-O system [1], a great deal of experimental and theoretical effort have been made to clarify the nature of the Cu-based oxides. In order to elucidate mechanism of the high-Tc superconductivity, discovery of a new type of superconductor is no doubt of great importance. Recently, Akimitsu et al. found a new oxide superconductor in the Nd-Ce-Sr-Cu-O system [2]. Soon after their discovery, the superconducting phase was isolated and identified [3]. It has a tetragonal cell with space group P4/nmm and has a structure closely related to but different from the K2NiF4− or T'-Nd2CuO4− -type structure. Although, Tc of the Nd-Ce-Sr-Cu oxide is not so high (ca. 20 K) compared with the 1–2–3 or Bi(Tl)-based superconductors, it has aroused interest widely due to a very simple crystal structure. In this article, I will discuss superconductivity and crystal chemistry of the Nd-Ce-Sr-Cu oxide. Also, various compounds isostructural to it will be presented.