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