Looking back on five volumes of Progress in Low Temperature Physics, it seems worth comparing this latest volume with the earlier ones, and thus draw attention to the trends which emerge. It is immediately evident that chapters in the early books were shorter; then they averaged 20-25 pages, whereas the present average approaches twice this figure. To maintain a reasonable size, it has therefore been necessary to decrease the number of chapters per Volume. Thus, in the rapidly expanding field of Low Temperature Research, succeeding Volumes in this series cover an ever-decreasing fraction of the body of knowledge.