This dissertation examines the development of critical colleagueship among five secondary science teachers in a semester-long video club. The design of the video club was intended to promote a focus on student thinking and experimentation with elements of ambitious science teaching. Over time, participants sustained a focus on interpreting students’ disciplinary thinking using evidence and began to problematize aspects of instruction related to making student thinking visible. Some participants attempted to change instruction to gain greater access to students’ disciplinary thinking while others did not. Efforts to experiment with instructional practice appeared related to alignment between participants’ learning goals and curricular contexts and the goals of the professional development design. Features such as framing activities, types of artifacts used, and facilitation, interacted differently over time to influence participant learning. Analysis revealed various tensions among the elements of the learning ecology that influenced participation. Findings from this study contribute to what is known about the importance of skilled facilitation as part of a learning ecology (Cobb, Confrey, diSessa, Lehrer, & Schauble, 2003) and has implications for the design of site-based professional development with secondary teachers.