Background/Context This article contributes to the literature on how teachers learn on the job and how schools and districts can support teacher learning to improve student learning and incorporate changing standards and curricular materials into instructional practices. The findings in this study are relevant to the implementation of ambitious mathematics instruction reform through changing teachers’ knowledge and instructional practices. Focus of Study This study examines how middle school teachers’ networks influence their mathematical knowledge for teaching (MKT) and instructional practices. We also examined how mathematics coaches’ expertise, in the form of MKT, plays a role in augmenting the extent to which teachers learn through interacting with close colleagues. Research Design The article draws on data from a larger NSF-funded study in four large, urban districts that responded to accountability pressures by attempting to implement ambitious mathematics instruction aligned with the recommendations of the National Council of Teachers of Mathematics (NCTM) and by supporting implementation with a significant investment in teacher learning. The analysis in this paper involves 89 focal participants who were middle school mathematics teachers in 29 schools, the focal participants’ close colleagues, and their instructional coaches. Measures include mathematics teachers’ professional networks, MKT, classroom practices, individual background characteristics, and school factors. We used hierarchical linear models with cross-level interaction effects and in-depth sensitivity analyses of the effects of close colleagues and coaches. Findings/Results Our results show that changes in teachers’ instructional practice were positively related to their access to instructional expertise through interactions with close colleagues. But, we did not find a similar significant relationship between changes in teachers’ MKT and access to their close colleagues’ MKT expertise. Rather, coaches’ MKT expertise positively moderated the extent to which teachers learned MKT from their close colleagues through seeking advice on teaching mathematics; that is, having an expert coach in the school enhanced the MKT learning opportunities that teachers had from interacting with close colleagues. Conclusions/Recommendations Results from this study shed light on how to support teachers’ on-the-job learning and successfully implement ambitious instructional reforms in schools. It is important for schools and districts to consider ways to encourage the development of teacher networks that can promote instructional changes. For example, schools and districts can purposely provide common planning time and common workspaces that facilitate sharing expertise among teachers. They can also support teachers with instructional coaches who have content expertise and know how to facilitate interactions among teachers.