With the publication of Volume 22 of Advances in Computers we continue the presentation of subjects which are not only timely but also of long-range interest to the computer and information science community. Contributions have been solicited from well-known experts who recognize the importance of providing review or tutorial articles covering their areas of expertise. Advances in Computers permits the publication of survey-type articles which are written from a relatively leisurely perspective and which may be somewhat lengthier than other publication media are willing to accept. It is thus possible to treat topics both in depth and in breadth. Volume 22 continues a serial publication which began in 1960. In these 22 volumes many articles have had a significant impact on the development of computers and their applications. That this serial has continued for these many years is a tribute to its reputation and the regard in which it is held.