To address the important issue of effective mathematics instruction, professional learning groups have been explored as a means to provide teachers the support they need to continue developing professionally. This research focuses on a narrative case study of one professional learning group to explore how the discussions provided the needed support for mathematics teachers in using research-based practices in their classrooms. A discussion of the impact that the teachers' knowledge of mathematics and teaching, as well as their beliefs about mathematics teaching and learning were explored to examine the impact knowledge and beliefs have on participation in and implementation of professional development. A model was created, and discussed, in order to provide a description of characteristics and dynamics needed in a professional learning group in order to support teachers in their mathematics teaching development.