The Scientist in the Classroom Partnership (SCP) is a unique program that partners scientists-in-training with classroom teachers to co-teach one day per week in elementary and middle schools. This program has shown positive impacts on all participants, and has the potential to promote professional learning for both the scientists and teachers. The Collaborative Apprenticeship model has been suggested as a unique situated professional development approach for both novice and mentor teachers. The current study examined factors that support the SCP program as a unique Collaborative Apprenticeship (CA) model in which scientists enter the partnership as novice teachers but expert scientists, and teachers bring expertise in pedagogy but are novice scientists. Over the course of participation in the program, the teachers gain scientific content knowledge and grow in their practice, and scientists gain pedagogical skills. Through this unique collaboration, both the scientists and the teachers move through the phases of the CA model (Introduction, Developmental, Proficient, and Mastery) so that by the end of their participation, each grows in a different area of expertise toward mastery.