Abstract
It has long been recognized that in order to build a good system in which a person and a machine cooperate to perform a task it is important to take into account some significant characteristics of people. These characteristics are used to build some kind of a "user model". Traditionally, the model that is built is a model of a canonical (or typical) user. But often individual users vary so much that a model of a canonical user is insufficient. Instead, models of individual users are necessary. This article presents some examples of situations in which individual user models are important.