Differentiated instruction is a widely held practice used by teachers to provide diverse learners with complex learning opportunities in the area of mathematics. Research on differentiated instruction shows a multitude of factors that support high quality instruction in mixed-ability elementary classrooms. These factors include small-class size, extra time and resources that allow for a highly individualized approach to instruction, teacher commitment, and subject-matter competency in mathematics. The literature also points to the role of leadership in providing a greater investment in teachers' professional development to improve teacher practice. This in-depth case study is an attempt to understand what resources and supports are needed to build teachers' capacity to effectively implement differentiated instruction to meet the challenges of teaching diverse learners found in classrooms today. For this study, I developed a conceptual framework to map the territory for understanding what personal factors and organizational conditions facilitate high quality implementation of differentiated instruction. Drawing from the literature, I identified specific personal factors and organizational conditions to study. I incorporated these criteria when I observed and interviewed the nine participants selected for this study. During the course of the study, I investigated across two different school environments, a high wealth and a low wealth district, and the dimensions that shaped the practices identified as high quality implementation of differentiated instruction. Overall, this framework allowed me to examine the differences in teacher practice that are not currently found in the literature. The findings of the study indicate several personal dimensions that distinguish a high implementer from a low and average implementer of differentiated instruction in mathematics. The dimensions are: a willingness to forge ahead and overcome obstacles; willingness to grow professionally and improve practice; strong competency, capability, and confidence teaching the subject matter; and ability to implement complex instruction in a variety of situations. All these coupled with prior teaching at the same grade level and strong classroom management skills increased the level of implementation quality. But even the high implementers made it clear that organizational conditions specifically, the pressure of daily work demands, put severe constraints on their ability to differentiate instruction in their classrooms. No amount of skill and determination or ample organizational resources could overcome these constraints.