The problem of the selection of first wall materials in a fusion device is discussed in view of core plasma performance as well as reactor engineering. For control of the core plasma condition, the material's characteristics especially as impurities, are mentioned. From a reactor engineering viewpoint, the speed of erosion is particularly explored. It is emphasized that, in order to satisfy both viewpoints, the design of the boundary plasma for the control of plasma — wall interaction is very important, and advanced studies on it would extend the scope of material selection.