Modeling Instruction holds the potential for transforming science instruction and improving student achievement. Key to the success of Modeling Instruction, however, is the fidelity of implementation of its curriculum. This qualitative study examined the impact of Modeling Instruction professional development on participating teachers' instructional practices. Through classroom observations and interviews, participants provided insight into challenges that impact fidelity of implementation. Participants for whom the professional development had no impact shared an additive view of the professional development along with external challenges that represented a misalignment between participants' goals and those of Modeling Instruction. Participants within the medium impact group held either transformative or additive views of the professional development. These participants described external challenges, which were closely related to their internal challenges. These internal challenges represented a cognitive dissonance that led to low fidelity of implementation. Finally, participants within the high impact group held a combination of both transformative and additive views of the professional development. These participants felt that they were reform-oriented prior to beginning the professional development, although the observations for two of the participants did not reveal this. The remaining participant's observation confirmed that she indeed practiced reform-oriented instruction prior to beginning the professional development, which likely supported her high fidelity of implementation. Implications for professional development providers are offered.