'Now . . . in the Analytical Engine I had devised mechanical means equivalent to memory. ' For the past twenty-five years or so, scientists and engineers have been endeavouring to realize in new technologies the claim made by Charles Babbage in his memoirs over a century ago. The modern computer industry depends to a very large extent on the success of their efforts. In this book we discuss the wide variety of techniques which have been used and are being developed to meet the range of requirements for digital storage systems in computers and other applications. The book has been written as a guide for the designer of any system employing digital techniques, firstly to guide him in his choice of store for differing applications and, secondly, to give him an apprecia tion of the problems which confront the engineer designing storage systems. Technology never stands still and developments in recent years have, of necessity, greatly increased the amount of material included in this second edition. The opportunity has also been taken to reorganize the contents and more emphasis has been given to those developments which have had, or which are likely to have, the greatest effect on computer development. Brief descriptions of obsolete or obsolescent systems have been retained, both as a warn ing to designers of the problems likely to be encountered in develop ment and to demonstrate how changes in technology can give a new impetus to old designs.