This dissertation addresses the challenge of designing software for the changing needs of learners by demonstrating an approach to the design of scaffolding in educational software. The approach, Guided Learner-Adaptable Scaffolding (GLAS), proposes that software be designed with learning supports that are fadeable by learners, with guidance from the software to help learners make fading decisions. A set of guidelines were proposed for the design of GLAS in interactive learning environments. In order to test these guidelines, they were applied to the design of a tool, TheoryBuilder, that supports learners building and testing dynamic models of complex systems. TheoryBuilder was tested in classroom use by approximately 95 9th grade students who used the tool three times during a school year. Eight focus students were videotaped throughout their use of the software in all three tasks, in order to record in depth their usage of and reactions to the software for qualitative evaluation. The models created by all of the 95 students in the class were also collected in order to compile quantitative statistical information about the students' changing use of scaffolding and fading mechanisms over time. Analysis of the data showed that TheoryBuilder generally achieved its goal of providing scaffolding for the modeling task. The scaffolding made the modeling task initially accessible, and students were able to fade scaffolding as they developed expertise. Students built larger and more complex models over time, fading scaffolding to use more advanced features and to turn off help that they no longer needed. The guidance provided with the scaffolding also appeared to help the students make informed decisions about fading scaffolding. The GLAS design guidelines were analyzed and revised in light of the data, and are offered as guidelines for the development of software that provides both accessibility and support for the development of expertise.Ph.D.Applied SciencesComputer scienceEducationEducational technologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/132202/2/9959821.pdf