This paper reports on a series of four studies carried out over a period of four years. These related studies were clinical and qualitative as they investigated middle and high school students' development of geometric thought, particularly as it related to three- dimensional visualization. The studies were carried out in the constructivist pedagogical style. The Geometer's Sketchpad (GSP) was instrumental in all of these studies and it was this environment in which the subjects learned to increasingly develop their visualization abilities as they solved challenging geometry problems. Platonic solids, Archimedian solids, other geometric solids, and the mathematical relationships related to these solids were the objects of study. Students' van Hiele levels of geometric thinking were monitored as they constructed dynamic GSP representations of these solids to conduct geometric investigations. A fundamental finding was that GSP provided opportunities to have a distinct positive affect on students' learning of three-dimensional geometry. (Author/NB)