Research on teacher learning consistently documents the disjuncture between the practices beginning teachers encounter in university teacher preparation courses and those they reencounter in the K-12 classrooms in which they learn to teach. As preservice teachers enter teaching, they gravitate toward conventional K-12 practices, dismissing those endorsed by the university as impractical. In this article, the authors delineate the concept of horizontal expertise and document how its production and use can address this “two-worlds pitfall.” Drawing on the authors' work creating a cross-institutional collaborative, they identify three processes central to the production of horizontal expertise in teacher education: the exchange of tools, the negotiation of social languages, and argumentation. They then trace its use across the university and school settings to show how horizontal expertise can rescript mentoring and expand dialogic practices in the university. The authors conclude by identifying the challenges of developing horizontal expertise in teacher education