The process of implementing intensive reading interventions using data-based decision-making (DBDM) becomes increasingly challenging as students move into the secondary grades and reading tasks correspondingly become more complex. This article provides teachers with guidelines to support effective implementation of DBDM for students with or at risk for reading disabilities in the secondary grades. Specifically, this article presents four steps for secondary teachers to follow within the context of a reading intervention to decide when instructional changes are needed based on progress-monitoring data. Diagnostic assessment is explained to determine students' strengths and weaknesses in order to target instruction accordingly. A case study is included throughout to demonstrate application of the steps as well as supplemental materials to help teachers implement this practice in their classrooms.