The University of Denver and its staff members deserve much credit for organizing and operating this Denver X-ray Conference year after year, for there seems to be no doubt that it and the yolumes that result from it are filling a need. The interests covered by the papers at one of these conferences vary from year to year and as a whole cover a wide spread of topics. This is as it should be. Old problems that have been with us for many years are being attacked again with new and more effective tools, new problems are continually arising, and new methods of great power are being developed. These developments are occurring in each of the fields covered, as may readily be seen by a glance at this twelfth volume and other recent volumes of this series. It seems clear that the policy of having these conferences and these volumes cover a wide field rather than a single one such as, for example, structure determination, or fluorescence analysis, is a policy that meets with general approval and should be continued. I understand there is every intention to do so. C. S. Barrett It is customary to acknowledge in each volume the invited session chairmen of the three-day meeting. They and the sessions at which they presided (21-23 August 1968) were as follows: CRYSTALLOGRAPHY AND DIFFRACTION. C. S. Barrett, University of Chicago, Chicago, Illinois. METHODS AND THEIR APPLICATIONS. B. C. Giessen, Massachusetts Institute of Technology, Cambridge, Massachusetts.