This i3 scale-up study examined the implementation and effectiveness of a 3-year literacy intervention developed by the Children's Literacy Initiative (CLI). The study was a school-level cluster randomized controlled trial conducted in 55 elementary schools from four states. Implementation results showed a high level of fidelity of intervention implementation across the treatment schools in the four urban school districts that had not previously worked with CLI. Intent-to-treat analyses of observation data showed that the intervention's effects on classroom literacy environment and teacher practices were not statistically significant in Year 1. By Year 3, however, treatment teachers who had taught in intervention schools for all 3 years were rated significantly higher in both the quality of classroom environment and the quality of literacy instructional practices than their peers in control schools. Analyses of student achievement data indicated that the CLI intervention had no statistically significant impact on students' reading achievement in any of the 3 intervention years. This held true for the overall student sample and for grade-specific subsamples, based on the overall literacy test score and for literacy subtest scores. In addition, differential impact analyses revealed that the impact of the CLI intervention on students' literacy achievement did not differ significantly by the level of students' baseline achievement. Further evidence from exploratory analyses showed that the intervention had no statistically significant impact on the English proficiency of English learner students or on teachers' knowledge of beginning reading practices. Because of the high amount of teacher attrition, an additional exploratory analysis that restricted the sample to only students of teachers who were stable across all 3 years in both conditions. There were positive and statistically significant effects for students in the CLI intervention condition who had stable teachers (teachers who had greater opportunity for intervention exposure) and who entered in Grade 1 and progressed to Grade 3 during the course of the study.