Students with reading disabilities require instruction and intensive intervention targeted to their current learning needs. Through data-based instruction, educators can monitor student's performance and make adjustments to instruction to better address student needs using both progress monitoring and diagnostic assessment. This article provides educators with a systematic approach to using error analysis of oral reading fluency data as a diagnostic tool to guide instructional decision-making. Additionally, it examines potential causes of word reading errors--such as phonemic, orthographic, morphemic, or high-frequency word challenges--and offers educator strategies for adapting and intensifying instruction based on individual student error patterns.