Background/Context: Reading achievement declined drastically during the COVID-19 pandemic (Bailey et al., 2021). One report suggests more than one-third of fourth-grade students cannot read at a basic level and access to effective literacy instruction is particularly limited for minority students and students with learning disabilities (Ellis et al., 2023). Closing the COVID gap and accelerating learning has been a priority; it is essential to identify and implement evidence-based programs and interventions that can meet these goals. Recent emphasis on the science-of-reading (Duke & Cartwright, 2021; Shanahan, 2020) provide further impetus. Enhanced Core Reading Instruction (ECRI) is a reading intervention aligned with structured literacy principles and the science of reading. Prior "efficacy" studies meeting ESSA's highest definition of "evidence-based" have shown positive impacts of ECRI on foundational reading outcomes (Baker et al., 2015; Fien et al., 2015; Smith et al., 2016). A recent study found students in ECRI classrooms outperformed comparison students on DIBELS (effect sizes of g = 0.25-0.41)and WRMT measures (g = 0.41-0.48; Fien et al., 2021). The current "effectiveness replication" study is the first to evaluate ECRI under routine implementation conditions and in a post-COVID context. It examines ECRI's impacts on the foundational reading skills of first-grade students with or at risk for reading disabilities or dyslexia. Research Questions: RQ1: To what extent is ECRI implemented with fidelity? RQ2: To what extent does comparison-group practice differ from ECRI? RQ3: What is the causal impact on distal standardized measures of alphabetic understanding and word recognition for students eligible for Tier 2 intervention? RQ4: What impact is evident on less distal measures? RQ5: How are ECRI effects moderated by student baseline characteristics, demographics, or mediated by implementation features? Setting: The 20 schools in Cohort 1 of the study are in ten public districts in the mid-Atlantic. One school is in a large city, 13 in suburbs, and six in distant towns or rural areas (see Table 1). Participants: In SY2022-23, adult participants were 100 first-grade teachers or reading interventionists (see Table 2). Student participants included the 1536 first-graders we screened. Intervention: ECRI is a comprehensive approach to early reading instruction and intervention for K-2 students. ECRI is designed for schools using a multi-tiered system of supports (MTSS-R; Gersten, 2009) and is used with core reading programs. Instruction addresses foundational reading (phonemic awareness, word reading, fluency), vocabulary, and comprehension skills. Tier 1 whole-class instruction (prescribed as 90 minutes daily) is characterized by explicit, scaffolded instruction and practice opportunities. Tier 2 (30 minutes) is highly aligned with Tier 1 and involves pre-teaching content to small student groups. ECRI also involves screening and progress monitoring, training, and coaching (see Figure 1). Research Design: We evaluate ECRI effectiveness with a cluster randomized controlled trial. Cohort 1 involves 20 schools randomly assigned to condition. We conducted universal screening of foundational reading skills on all first-grade students with DIBELS 8. Students eligible to receive Tier 2 intervention (i.e., scored above the 10th but below the 40th percentile) formed the analytic sample. We also conduct an implementation study (see Figure 3). Data Collection: SY2022-23 data are complete. SY2023-24 data collection will conclude in May. For the implementation study, we collected: (1) 20-minute "survey to teachers and interventionists" to gather their implementation and perception of ECRI. N = 77 surveys in SY2022-23; (2) "Tiered Fidelity Inventory for Reading" (St. Martin et al., 2015) measures the extent to which school personnel implement essential features of an intervention. N = 20 surveys; (3) "Observations" of Tier 1 and Tier 2 instruction. Instruments consist of one measure of fidelity to ECRI and three measures on the use of evidence-based practices (Doabler & Nelson-Walker, 2009; Fien et al., 2015; Nelson et al., 2013; Smolkowski & Gunn, 2012; Smith et al., 2016). N = 70 observed periods. For the impact study, we collected: (1) Pre- and post-test data using three DIBELS 8 and four WRMT-III subtests. In SY2022-23, we screened 1536 students, identifying 283 students for Tier 2 (i.e., our analytic sample). We double-coded [approximately]10% of assessments, achieving interrater reliability between 0.83-1.00 across subtests. (2) Administrative records, including attendance, student's age, grade, gender, race/ethnicity; English learner, special education, and FRPL status. Data Analyses: For RQ1-2, we generate descriptive statistics. Analyses for RQ3, our primary confirmatory question, has been registered with the Registry of Efficacy and Effectiveness Studies (REES). For the causal impact on student outcomes, we use a hierarchical model with students nested in Tier 2 group and school, with fixed effects for the treatment indicator, student and group-level covariates and randomization block. This model will be extended to explore RQ5 by interacting the moderator/mediator, already part of the vector of covariates, with the treatment indicator. Preliminary Findings: We present preliminary findings from SY2022-23. Data from 2023-24 will be pooled with SY2022-23 data, providing more power to detect effects. Full findings will be available early fall. RQ1-2. ECRI was implemented with strong fidelity (M = 0.88 on ECRI Observation Fidelity scale, ranging from 0-1); lesson materials were largely used and routines followed. Observations suggest a positive, though not statistically significant, difference in favor of ECRI on general features of instructional quality (see Table 3). ECRI classrooms provide more explicit instruction (M = 0.70 ECRI; M = 0.38 for comparison); three specific practices are significantly different (Table 4). And there are statistically significant differences in student-teacher interactions (Table 5). Survey responses suggest ECRI teachers more strongly adhere to the provided program (e.g., ECRI), found it more desirable and professional development more helpful compared to comparison school teachers. RQ3-4. Results of preliminary analyses that control for baseline DIBELS and WRMT scores provide positive signals of ECRI effects on the foundational reading outcomes, though these are not (yet) statistically significant. For WRMT Word Identification subtest, the estimated effect is equivalent to the median ECRI student outperforming 57% of comparison group students. For the Word Attack subtest, it is 62%. These are our primary study outcomes. Estimates for other measures are also positive (see Table 6). RQ5. We are exploring potential moderating effects of student baseline characteristics and implementation features.