This chapter focuses on the nature of “the object” modeled by a computer-based learning system. The general definition of a model as something that an “observer” uses to understand the “object” modeled indicates that the notion of modeling permeates the computer-based learning system design activity. In contrast, one of the basic assumptions of constructivist philosophies of knowledge is that knowledge cannot be objectively defined and statically represented. The original rationale for needing student models was that individual students are so different that their needs cannot be anticipated at design time. Similarly, the properties of a process of learning fixed at design time; indeed, it would be self-contradictory for a constructivist to claim that designers can know such properties in advance. A corollary of this emphasis on the process of learning is an increased focus on the time and context in which events occur.