The development of the human-computer interface since the early 1980s has brought about a fervor of generating models of human behavior, models of machines, and models of the interaction between the two. The literature is filled with the issue of model and modeling to such an extent that it is no longer clear what is what. If a model were a mirror, the human-computer interface would be a hall of mirrors and just as intriguing, deceptive, and misleading. Nevertheless, models form a bridge across the gu lf that separates the human mind and the machine. This chapter is an attempt to survey models and modeling approaches at the human-computer interface. This review does not promise to be either complete or formally accurate owing to the relentless emergence of new developments and the inherent flux of n ew ideas within ther field. Nevertheless, it serves to touch upon as many of the aspects, issues and developments in models and modeling as possible.