The Editors take pleasure in presenting Volume 13 of this annual review series, consisting, as usual, of author­ itative reviews of timely developments in the technical fields of nuclear engineering, science, and teechnology. No one in the community we try to serve in a post­ Harrisburg era will need convincing of the relevance of the first two items to be mentioned from the volume. Instru­ mentation for two-phase flow measurements, by Banerjee and Lahey, has applicability in the engineering research labor­ atory and to power reactors; the U. S. LWR still remains the dominant power reactor type and seems likely to retain its hold if only through the capital of existing plants this century. Messrs. Bohm, Closs, and Kuhn, however, have a longer time scale to respect as they view for us the prospects of nuclear waste disposal from a European viewpoint. They bring out nicely the political aspects that cannot be divorced from technical considerations in this area, or in the more militant terms of confrontation, in this arena, perhaps. We are pleased to carry in this volume two complemen­ tary papers on mathematical methods in nuclear engineering.