The objective of this paper is to present attitudinal and cognitive beliefs of two preservice secondary mathematics teachers about teaching geometric transformations (GTs) using Geometer's Sketchpad (GSP). The study comprised ten task-based interviews, a series of five each with two participants, senior undergraduate preservice mathematics teachers, at a medium-sized public university in the Rocky Mountain Region of the United States of America. Radical constructivist grounded theory (RCGT) was the theoretical framework used both to guide the study process as well as to integrate the research participants, the researcher, research process, and the data. The results of the study include in vivo categories of the participants' beliefs associated with attitude and cognition of teaching GTs with GSP. These include developing confidence with GSP, efficiency of using GSP for teaching GTs, exploring GTs with GSP, conjecturing GTs using GSP, supporting and engaging students in learning GTs, and understanding GTs with GSP. Some pedagogical and policy implications of these categories are also addressed.