The No Child Left Behind Act of 2001 created the Reading First program to help ensure that all students could read at or above grade level by the end of grade 3. The program promotes practices recommended by the National Reading Panel for early reading instruction, highlighting essential components of reading instruction. As required by the legislation, the Reading First impact study examined instruction and student reading outcomes in 248 schools (about half Reading First and half not) in 17 districts and 1 statewide program to address three main questions: (1) What is the impact of Reading First on student reading achievement? (2) What is the impact of Reading First on classroom instruction? and (3) What is the relationship between the degree of implementation of scientifically based reading instruction and student reading achievement? The study collected observational data on reading instruction in grades 1 and 2 and assessed student reading comprehension in grades 1 through 3 over three school years: 2004-05, 2005-06, and 2006-07. The study also assessed students' decoding skills in grade 1 and surveyed school personnel about their reading programs in spring 2007. Reading First was not found to improve student reading comprehension in grades 1, 2, or 3, although it did improve first graders' skills in decoding unfamiliar words. It also produced changes in instructional practices, including the amount of time spent on essential components of reading instruction and the amount of professional development in reading instruction. The analyses are considered exploratory because they are based on correlational analysis. The study was not designed to provide a rigorous test of these hypotheses, so the results must be considered as suggestive. (Contains 1 note and 2 figures.) [For the full report, "Reading First Impact Study. Final Report. NCEE 2009-4038," see ED503344. For the Executive Summary of the report, "Reading First Impact Study. Final Report. Executive Summary. NCEE 2009-4039," see ED503345.]