A current theme in the educational literature is that of “cognitive tools” (Pea, 1985; Perkins, 1985; Salomon, Perkins, & Globerson, 1991). The metaphor implies that there are tools that can assist learners to accomplish cognitive tasks. There are at least 4 types of cognitive tools that can be identified by the functions they serve. These 4 tools can: (a) support cognitive processes, such as, memory and metacognitive processes; (b) share the cognitive load by providing support for lower level cognitive skills so that resources are left over for higher order thinking skills; (c) allow the learners to engage in cognitive activities that would be out of their reach otherwise (Pea, 1985; Olson, 1988); and (d) allow learners to generate and test hypotheses in the context of problem solving. These 4 tools are not mutually exclusive. Cognitive tools that serve these functions have been incorporated into 2 computer systems that are addressed in this chapter. The first is Sherlock I, a computer-based learning environment for avionics troubleshooting (Lajoie & Lesgold, 1989; Lajoie, Lesgold, et al., 1989; Lesgold, Lajoie, Bunzo, & Eggan, 1992). The second is Bio-world (Lajoie, 1990, 1991), a computer-based learning environment that provides high school biology students with practice at diagnosing infections. My chapter describes these systems, the cognitive tools they incorporate, plus empirical evidence that such tools support learning.