This document reports on the impacts on student achievement for four supplemental reading curricula that use similar overlapping instructional strategies designed to improve reading comprehension in social studies and science text. Fifth-grade reading comprehension for each of three commercially-available curricula (Project CRISS, ReadAbout, and Read for Real) was not significantly different from the control group. The fourth curriculum, Reading for Knowledge, was adapted from Success for All for this study, and had a statistically-significant negative impact on fifth-grade reading comprehension. The study is based on a rigorous experimental design and a large sample that includes 10 districts, 89 schools, 268 teachers, and 6,350 students. During the first year of the study, it was found that over 90 percent (91 to 100 percent) of treatment teachers were trained to use the assigned curriculum, and more than half (56 to 80 percent) reported that they were very well prepared by the training to implement it. Over 80 percent (81 to 91 percent) of teachers reported using their assigned curriculum. Classroom observation data showed that teachers implemented 55 to 78 percent of the behaviors deemed important by the developers for implementing each curriculum. Scores on the three reading comprehension assessments were not statistically significantly higher in schools using the selected reading comprehension curricula. Impacts were correlated with some subgroups defined by student, teacher, and school characteristics. Appendixes include: (1) Random Assignment; (2) Flow of Schools and Students through the Study; (3) Obtaining Parent Consent; (4) Implementation Timeline; (5) Sample Sizes and Response Rates; (6) Creation and Reliability of Classroom Observation and Teacher Survey Measures; (7) Estimating Impacts; (8) Assessing Robustness of the Impacts; (9) Key Descriptive Statistics for Classroom Observation and Fidelity Data; (10) Study Instruments; and (11) Unadjusted Means. (Contains 91 tables, 8 figures, and 78 footnotes.)