Crossing risky boundaries: Learning to authentically and equitably co- teach through design and practice Jacob Hackett a, * , Megan Bang b , Arielle Goulter c , Maritess Battista c a Georgia State University, College of Education & Human Development, 30 Pryor Street #633, Atlanta, GA, 30303, USA b Northwestern University, Evanston, IL, USA c Highline Public Schools, Highline, WA, USA highlights  Co-teaching should be viewed as an activity system, shifting from sets of practices, checklists or procedures.  Activity systems consider co-teacher beliefs, psychological safety, instructional practices and practitioner learning.  Participants sharpened their language and co-teaching practice by designing 2 novel and adaptable co-teaching tools.  The Co-teaching Implementation Framework is a powerful initial step towards a nuanced theory of the co-teaching system. article info Article history: Received 6 November 2018 Received in revised form 1 May 2019 Accepted 31 July 2019 Available online 10 September 2019 1. Collaborative (Co-) teaching: supporting inclusive classrooms American classrooms have recently seen significant increases of students with learning differences educated in inclusive general education settings (U.S. Department of Education, 2014). This trend e certainly a positive leap forward in equity and disability rights e has demanded a tremendous amount of teacher knowledge and preparation to serve student learning needs and achieve mean- ingful outcomes. In response, school districts have increasingly embraced a form of instructional delivery e collaborative (co-) teaching e to expand those resources (Friend & Bursuck, 2011; Scruggs & Mastropieri, 2017). Essentially, co-teaching partners a general and special educator together in the same space to plan, design and deliver instruction in their inclusive classroom (Bang & Vossoughi, 2016). Though no definitive rates of prevalence are available, the foremost advocacy and lobbying organization for exceptional learners, special educators, and special education policy ethe Council for Exceptional Children (CEC) e recommends districts and administrators employ co-teaching. Ploessl and Rock (2014) also attest to its occurrence as they probed administrators and teachers for the most effective method to facilitate student learning in in- clusive settings, as co-teaching was frequently offered. CEC's endorsement and Ploessl and Rock's findings underscore co- teaching's popularity, but current theory and empirical research on student and teacher outcomes is limited, highly variable, and doesn't necessarily substantiate co-teaching's effectiveness (Cook, McDuffie-Landrum, Oshita, & Cothren Cook, 2011; Murawski, 2006; Pancsofar & Petroff, 2013). Structurally, co-teaching can embody multiple configurations, including: (1) one teach, one assist; (2) station teaching; (3) parallel teaching; (4) alternative teaching; (5) team teaching; and (6) one teach, one observe (see Cook et al., 2011 for descriptions; Kloo & Zigmond, 2008). Each arrangement has distinctive affordances and constraints, particularly with respect to supporting instruc- tional delivery, curriculum learning goals, and classroom environ- ments. Since each model has a unique purpose, it is critical that practitioners deeply understand each model in relation to its implementation and develop sound rationales to select each model within the sequencing of their lessons and units (Friend, 2008; Friend, Hurley-Chamberlain & Shamberger, 2010). The practitioner learning occurring as co-teachers negotiate the intentional selec- tion of each model in their practice could be a vital contribution to co-teaching research by elevating an expansive approach. An overemphasis and prevalence of traditional reductionist co- teaching research, which can neglect personal and instructional * Corresponding author. E-mail addresses: Jhackett2@gsu.edu (J. Hackett), megan.bang@northwestern. edu (M. Bang), arielle.goulter@highlineschools.org (A. Goulter), maritess.battista@ highlineschools.org (M. Battista). Contents lists available at ScienceDirect Teaching and Teacher Education journal homepage: www.elsevier.com/locate/tate https://doi.org/10.1016/j.tate.2019.102889 0742-051X/© 2019 Elsevier Ltd. All rights reserved. Teaching and Teacher Education 86 (2019) 102889