The papers in this symposium report on findings from the Chicago Literacy Initiative: Making Better Early Readers (CLIMBERs) study, a five-year, large-scale study designed to test the effectiveness of Breakthrough to Literacy (BTL), a comprehensive early literacy curriculum, in preschool and Kindergarten in the Chicago Public Schools. In all, 44 schools were recruited and randomly assigned to either a treatment condition (BTL) or a “business-as-usual” control condition. In all, 67 preschool classrooms and 108 Kindergarten classrooms participated. Treatment and control classrooms in both preschool and Kindergarten were observed at baseline in the first year and in the following two springs. Two cohorts of children in both grade levels were assessed at baseline and at the end of the school year. The hypothesis was that children in the treatment classrooms would outperform those in the control classrooms on measures of early language and literacy. This paper presents findings on the impact of Breakthrough to Literacy (BTL) on the early literacy and language development of two cohorts of pre-kindergarten children and on teachers’ instructional behaviors at the end of the first year (4-5 months of implementation) and at the end of the second year of implementation. The pre-kindergarten sample consists of data from 42 schools, 67 classrooms, and a total of 1823 students from two cohorts (2004-2005 and 2005-2006 school years).