This paper presents findings of an efficacy trial examining the effect of a multi-tiered instruction and intervention model on first grade, at risk students' reading outcomes. Schools (n = 16) were randomly assigned to the treatment or control condition. In the fall of Grade 1, students were assigned to an instructional tier on the basis of SAT-10 scores (>39th percentile = Tier 1; 16th-39th percentile = Tier 2). In both conditions, Students identified as at-risk (i.e., Tier 2; n = 267) received 90 minutes of whole group instruction (Tier 1) and an additional 30 minutes of daily small group intervention (Tier 2). In the treatment condition, teachers were trained to enhance core reading instruction by making instruction more explicit and increasing practice opportunities for students in Tier 1. In addition, at risk students were provided an additional 30 minutes, daily small group intervention with content that was highly aligned with the Tier 1 core reading program. Results indicate significant, positive effects of the intervention on students' decoding and fluent reading and substantive effects on total reading achievement. [This paper was published in "Journal of Learning Disabilities" v48 p602-621 2015 (EJ1076683).]