This study explored mathematics teachers' reasoning regarding their instructional design for technology use in teaching mathematics. Various types of qualitative data were obtained and analysed from a sample of 19 secondary and 28 primary preservice teachers. The findings showed that the purposes of technology use, teachers' conceptions of mathematics and its learning, their learning experience and knowledge of student thinking as well as mathematical cognition, and empirical inquiry were the main sources of teacher reasoning. Three levels of teacher reasoning were identified: descriptive, explanatory, and justifying. Our findings contributed to research on teacher professional development by identifying four types of technology use with a variety of pedagogical purposes, and formulating two main dimensions to characterise the three levels of teacher reasoning.