This report presents the findings from the Instructional Dimensions Study (IDS). The IDS examined the relationship between selected program characteristics such as instructional setting, time, content, and individualized instructional practices, and program effectiveness. Specifically, it assessed the effects of these program characteristics on the reading and mathematical achievement of low-achieving students in the early elementary grades. The major goal of the research was to provide information on classroom processes that effectively promote learning. In general, the study's results were positive about the effectiveness of compensatory instructional programs. First graders in the sample made average gains of 12 months in reading and 11 months in mathematics in the 7-month period between fall and spring testing. Third graders gained 8 months in reading and 12 months in math. Students in the first grade gained most when they received compensatory instruction in their regular classes. The results for the third grade were mixed. Students in individualized instruction programs made substantial gains in achievement. However, individualization, as defined in this study, is effective, but not uniquely so. Students' gains were shown to be clearly related to the content and intensity of instruction received. (Author/AM)