Background/Context: Spurred by changes in federal policy, Response to Intervention (RTI) has witnessed increased adoption across states and districts in the past several decades (Berkeley et al., 2020). Originating in the special education field, RTI involves a shift from a uniform instructional approach to a tiered instructional approach consisting of strong evidence-based core instruction for all students and increasing levels of support for students not making adequate progress (Fletcher & Vaughn, 2009). RTI is also meant to replace prior methods of identifying specific learning disabilities (SLD) by providing special education (SpEd) services only to the most persistent intervention non-responders (Vaughn & Fuchs, 2003). Although RTI has been scaled in thousands of schools over the past 20 years (Balu et al., 2015), little causal evidence exists on the impacts of RTI on student outcomes as it is brought to scale (Torgesen, 2009; Wanzek & Vaughn, 2011; Gilmour et al., 2023). Purpose/Objective/Research Question: In this paper, we leverage the staggered rollout of RTI across districts in a mid-sized western state using a difference-in-differences design to examine the effects of RTI adoption on students' disability identification and achievement outcomes in elementary school. Specifically, we ask the following research questions: 1. How does exposure to RTI impact students' identification for special education throughout elementary school? What are the effects on SLD identification? 2. How does exposure to RTI impact students' state standardized test scores? 3. Are there heterogeneous effects of exposure to RTI by race/ethnicity and socioeconomic status? Setting/Population/Participants/Subjects: The present study takes place in a mid-sized state in the western United States. We use the population of elementary students enrolled in public schools from 2007/2008 to 2021/2022 (average annual N [approximately equal to] 250,000). Across the panel, 1.5% of students identified as Native American, 4.5% as Asian/Pacific Islander, 2.5% as Black, 23% as Hispanic/Latine, 5% as Multiracial and 63% as White. Fifty-eight percent qualified for free or reduced-priced lunch, while 15% received SpEd services. Intervention/Program/Practice: Between the 2005 and 2018 school years, the Department of Education in this state rolled out RTI by providing training and technical assistance to cohorts of districts selected through a competitive application process. Selected districts received programming support to adopt RTI. In return, districts committed to fully implementing the RTI model; using RTI to identify SLD, adopting a research-based core instructional program, using universal screeners and progress monitoring tools, providing tiered interventions, and collecting student data. Notably, these efforts were all targeted towards improving student reading outcomes. There was some funding attached to the technical assistance and professional development provided. Still, districts had to be able to support the continued use of RTI without the state's financial support. Moreover, program staff provided formal assistance to schools in the first few years after they adopted RTI but faded these supports over time to help schools work towards independently maintaining implementation. Research Design: In this paper, we use a difference-in-differences design that leverages the staggered timing of participation in the RTI technical assistance program across districts. This design utilizes the trend in outcomes among untreated units to project the trend for treated units had they not received treatment. Treated units are then compared to this projected value as the counterfactual to estimate treatment impacts which, when certain assumptions are met (namely parallel trends and no anticipation), represent causal estimands (Roth et al., 2023). Data Collection and Analysis: Data come from state administrative records for all K-5 public school students which are then merged with publicly available data on the timing of district-wide RTI program participation to generate a unique individual-level panel data set. We estimate differences-in-differences designs using the Callaway & Sant'Anna (2021) estimator to examine average changes in student outcomes post intervention and how those impacts developed over time for schools experiencing more years of the policy change. Findings/Results: We find that RTI reduced special education identification rates by 1.4 percentage points (11%; p < 0.01) and SLD identification rates by 0.5 percentage points (15%, p < 0.01). Interestingly, we identify spillover effects on disability categories not directly targeted by the policy, with adoption leading to statistically significant decreases in the identification of speech or language impairment. Overall, these changes to disability identification did not accompany increases in average reading test scores, the academic area targeted by the policy, nor were there durable spillovers on math achievement or student discipline. Nevertheless, we can rule out average decreases in test scores greater than 0.03 standard deviations (SD), suggesting that test scores did not decline due to changes in disability identification. Finally, while there were null overall effects on achievement, we find evidence that Black students in RTI schools experienced reading test score gains of 0.15 SD (p < 0.001), suggesting that RTI adoption was equity-enhancing for some student populations. Conclusions: These results provide novel evidence on the impacts of a longstanding and understudied education policy reform as it was implemented at scale and contribute to conversations on how to structure school systems to meet the needs of students with and without disabilities. On balance, our findings indicate that RTI in the state under study achieved its intended disability identification outcomes and was at least partially successful in ameliorating inequities, highlighting the promise of the RTI model for effectively supporting students at scale.