DR. GERARD J. HOLZMANN is the principal designer of the SPIN system. Formerly Directory of Computing Principles Research at Bell Laboratories in Murray Hill, N.J., he recently joined NASA's Jet Propulsion Laboratory in Pasadena, CA, to help set up a new Laboratory for Reliable Software. Holzmann's earlier books include Design and Validation of Computer Protocols (Prentice Hall), and The Early History of Data Networks (IEEE CS Press).