This article describes a three-part empirical approach to understanding the development of expertise within the UNIX' operating system. We studied UNIX users with varying levels of expertise. The first part of our research attempted to ascertain the nature of their conceptualizations of the UNIX system. The second part measured users' performance in tasks requiring them to comprehend and produce UNIX commands. The third part was a longitudinal rather than cross-sectional analysis of the emergence of expertise. The conceptualization data suggest important differences in the models of UNIX structure formed by each group. Experts best represent the higher levels of the UNIX system; novices more fully represent the lower, more concrete levels of the system, including specific commands. UNIX users also differ markedly in performance, according to their history of use with the operating system. Only experts could successfully produce composite commands that required use of the distinctive features of UNIX (e.g., pipes and other redirection symbols), even though the intermediates and novices evidenced the component knowledge required for composite commands. This finding is somewhat surprising, inasmuch as these are fundamental design features of UNIX, and these features are taught in elementary classes. These data suggest, however, that these features can be used reliably only after extensive experience. The longitudinal data suggest that most subjects increased in expertise. However, expertise can decrease as a function of time, depending on system use. Those subjects who increased in expertise acquired the ability to produce the simple commands and represent the basic modules before they acquired knowledge of complex commands and advanced utilities. The nature of expertise is considered with respect to both system design and user characteristics, including users' conceptual models of system structure.