This study examined the effectiveness of two tiers of intervention for preventing early reading difficulties among kindergarten children who qualified as at risk of experiencing difficulty in the early stages of learning to read. Participating schools were randomly assigned to one of three treatment conditions: Professional Development for classroom teachers which served as a Tier 1 intervention, small group supplemental Intervention for children which served as a Tier 2 intervention, or both Professional Development for teachers and direct intervention for children. Three cohorts of children were followed from the beginning of kindergarten to the beginning of first grade. The first cohort served as a Baseline Cohort. The experimental treatments were instituted for the second cohort (Implementation Cohort). The third cohort served as a Maintenance Cohort which allowed us to evaluate whether effects of PD were still evident in the year following the implementation of the PD program. Data were gathered on both student achievement and classroom language arts instruction for each cohort. Outcomes suggested that all three treatments were very effective in reducing the number of at risk kindergartners who remained at risk for reading difficulties at the end of the school year although differences in the effectiveness of classroom instruction observed for the Baseline Cohort made it difficult to confidently compare the relative effectiveness of the three treatment conditions. However, comparisons between the Baseline and Maintenance Cohorts clearly revealed characteristics of classroom instruction that were associated with reductions in the number of children who qualified as at risk for reading difficulties at the beginning of first grade. (Contains 3 figures and 7 tables.)