In the present volume and in the preceding one we have stretched our normal pattern of reviews by including articles of more major proportions than any we have published before. As a consequence each of these two vol umes contains only three review articles. From the beginning of this series it has been our aim, as editors, to achieve variation in the scope, style, and length of individual articles sufficient to match the needs of the individual topic, rather than to restrain the authors within rigid limits. We feel that the two major articles of Vols. 5 and 6 are entirely justified and do not repre sent unnecessary exuberance on the part of the authors. The article by Michaudon on fission is the first comprehensive account of the developments in this subject, which have placed it in the center of the stage of nuclear physics during the past few years. The discovery of fission isomerism and its dramatic manifestations in the intermediate structure of the neutron cross sections for fissionable isotopes are among the most im portant and interesting events to occur in nuclear physics. These events came as a surprise, and reaffirmed that the strength of nuclear physics lies in the combination of ingenious experiments with simple ideas.
