Reading outcomes at a national level have remained stagnant for more than two decades. One reason why is that the field has struggled with how to address poor reading comprehension when reading words is not the problem. Another is that limited insight into the causes of poor comprehension performance is offered by traditional reading comprehension measures. This article describes the Multiple-choice Online Causal Comprehension Assessment (MOCCA), a measure of reading comprehension designed to assess the cognitive processes in which Grade 3 to 6 students engage when trying to comprehend as they read. In contrast to traditional reading comprehension assessments, MOCCA provides diagnostic information about how students who are struggling with reading comprehension cognitively approach the comprehension task. Multiple-choice Online Causal Comprehension Assessment scores are directly aligned to instructional decisions and are most useful when triangulated with other reading curriculum-based measures, for which this article provides two examples and a decision-making heuristic.