The purpose of this study was to understand how and why middle grades mathematics teachers did or did not fully participate in sustained, content-focused professional development. Qualitative data were collected from middle grades teachers who demonstrated varying levels of participation. The teachers identified opportunities to engage in novel ways with mathematics content as effective in improving their own mathematics understanding and, consequently, their own instruction. Participants who were more critical of the professional development felt the mathematics content was often beyond their specific grade-level. Factors identified as affecting teachers' participation in the training included administrative support and school-related responsibilities. Offering content-focused professional development designed to reflect the structures and characteristics established in research to most effectively provide teachers with needed knowledge and skills was not enough to ensure teacher participation.