This mixed-method study investigates the quality of teachers’ implementation of mathematical tasks and the nature of changes that occurred during an academic year with the support of a professional development program. Task implementation quality consists of three dimensions: total cognitive demand, attention to student thinking and intellectual authority. The study took place in the context of a 1-year professional development program, with the participation of four teachers in a private school. Data were gathered from class observations and interviews with teachers. Both quantitative and qualitative analyses indicated changes in the teachers’ practice. The major change was in the total cognitive demand in the implementation of tasks. An analysis of changes in cognitive demand level revealed fluctuations in the total cognitive demand of mathematical tasks, indicating that changes in teaching practice are complex and often nonlinear. The findings are discussed along with potential teacher-related factors. Recommendations are made for future research on professional development programs.

Keywords: Mathematical tasks, Task implementation quality, Professional development, Cognitive demand, Intellectual authority, Student thinking