Handheld computers are mobile, flexible devices that can provide real-time, one-to-one support for students from within the context of their learning activities. This paper describes the design of three learner-centered handheld tools used as part of a nine-month classroom study involving thirty-three eighth grade students. A review of related work identifies some of the challenges of building educational software within the constraints of handheld screens, and two broad design guidelines are synthesized to help address these challenges. The first design guideline focuses on decomposing the learning activity to identify salient tasks and the type of supports (or scaffolds) students need to engage in these tasks, then building separate handheld workspaces to support each task. The second guideline focuses on methods for implementing scaffolds within these task-based workspaces while preserving the usability of the overall handheld software.