This study investigated the actual learning processes of two preservice teachers as they explored geometry problems with dynamic geometry software and the effects of using the software on developing their mathematical reasoning and proof abilities. A constructivist teaching experiment was designed and implemented for the investigation. Through participating in the study, the subjects developed a new learning style--exploring problem situations through a learning process characterized by initial conjecture, investigation--more thoughtful conjecture, verification (or proof)--proof (or verification). They changed their conceptions of mathematics and mathematics teaching as well. The Geometer's Sketchpad (a dynamic geometry software package) was found to be an excellent teaching and learning tool that can enhance students' mathematical reasoning and proof abilities. (Author)