This research aimed to examine teachers' experiences whilst implementing a reform approach to mathematics teaching in an Irish primary school In particular, it aimed to examine the constraints and affordances pertaining to changing mathematics practice in this case study school The school engaged in professional development that focused on using an instructional framework (Hiebert et al, 1997) in the school-identified strand units of length and weight. Four classes were tracked throughout the study and each class acted as a subcase within the larger study. Data collected through lesson observations, teacher interviews, pupil work samples, and focus group interviews with pupils were used to compare teachers ’ experiences and to identify what supported and enabled them to change their practice, in addition to highlighting challenges that may have prevented such change. Findings were that shifts in practice were evident in the strand units of length and weight to different degrees in the four classes during the study. Teachers reported that tailored professional development, peer support, and the use and refinement of an instructional framework contributed to enabling them in changing their mathematics teaching. Factors that may have restricted the potential for embedding and enhancing this changing practice included a textbook dependent culture, teacher discomfort with facilitation, and possible limitations regarding the professional development. Possible implications of this study include the need for schools to have a number o f factors in place for the successful implementation of a reform approach to mathematics teaching including peer support, tailored professional development, and a focus on a specific area o f the mathematics curriculum. Furthermore, this study highlights the need for all stakeholders to consider textbook dependency, teacher facilitations skills, and the timing and content o f professional development when implementing a reform approach to mathematics teaching.