Background: In Canada, contemporary collaborative professional learning models for educators utilise multiple forms of evidence to inform practice. Commonly, two forms of evidence are prioritised: (a) research-based evidence and (b) classroom-based evidence of student learning. In Ontario, the integration of these two forms of evidence within professional learning is increasingly facilitated by "middle leaders"--school district level educators who support system change through sustained school- and classroom-embedded professional development with teachers. Purpose: This study investigated the role of the middle leader in supporting teachers' use of research- and classroom-based evidence within the context of collaborative professional learning. It was guided by the following research questions: (a) What is the capacity of middle leaders to use research- and classroom-based evidence within professional learning contexts? (b) What challenges do middle leaders encounter in using research- and classroom-based evidence within professional learning contexts? and (c) How do middle leaders develop their capacity for evidence use within professional learning contexts? Programme Description: The context for this study was a three-year networked professional learning project that included nine school districts in Ontario, Canada. An evidence-based collaborative inquiry approach was used to explore the common goal of improving mathematics teaching and learning across the nine districts. Participants: A purposeful sample of 30 middle leaders (i.e. system facilitators) and 54 teachers (36 Primary/Junior school teachers [student ages 4-11] and 18 Intermediate/Senior school teachers [student ages 12-18]) across the nine school districts was selected based on these educators' high degree of engagement in the project and willingness to participate in data collection. System facilitators were relative experts in mathematics instruction and provided differentiated and embedded support in mathematics pedagogy to school inquiry teams (i.e. school administrators and selected teachers), responsive to their local needs and goals. Design and Methods: A qualitative approach was used, comprising focus groups conducted at the end of each project year with study participants. Data collection yielded 27 system facilitator focus groups (nine per year) and 27 teacher focus groups (also nine per year), each transcribed verbatim. Findings: Standard thematic coding of focus group data was used to elucidate four emergent themes across system facilitators and teachers: (a) realising the need for data literacy in the construction of classroom evidence, (b) challenges to evidence use, (c) support enabling evidence use and (d) changing attitudes about evidence. Overall, findings indicated that both middle leaders and teachers required data literacy to use, successfully, classroom evidence within a collaborative inquiry. Although middle leaders encountered challenges to facilitating teachers' classroom evidence use in collaborative inquiry, they developed their own data literacy through regular cross-district middle leader learning sessions and targeted support from pedagogical and research experts. While both middle leaders and teachers shifted their attitudes and practices with respect to classroom evidence use within the project, this finding did not extend to their critical interrogation of research evidence sources (i.e. research literacy). Conclusions: Middle leaders play a central role in promoting evidence use within networked collaborative professional learning. Diverse strategies are needed to support middle leaders' use of both research- and classroom-based evidence throughout collaborative inquiry cycles, to inform and monitor classroom, school, district and regional impacts. While increased data literacy may enhance educators' use of classroom evidence, it may not directly impact their uptake of research to inform their collaborative inquiries.