Volume 19 of Advances in Computers continues the publication of in-depth articles of important current interest to computer and information science. Despite the currency of the topics, subjects have been chosen for inclusion largely because they are expected to be of interest over a considerable span of time. The Advances provides an opportunity for renowned experts to write review or tutorial articles covering a major segment of the field. The very nature of Advances in Computers permits the publication of longer survey articles that have been prepared in a somewhat more leisurely fashion. Volume 19 continues a long and unbroken series that began in 1960; in terms of computers, generations ago.