This study was conducted in order to identify the major reading, cognitive, and linguistic skills that contribute to individual differences in performance on the reading portion of the Florida Comprehensive Assessment Test (FCAT) at third, seventh, and tenth grades. The authors were also interested in understanding the primary deficiencies in skill and knowledge that are associated with below grade level performance on the FCAT (levels 1 and 2). The expectation was that this knowledge would be helpful in planning more effective methods to prevent performance difficulties on the FCAT and more effective remedial procedures for students who have demonstrated deficiencies on the FCAT. The results provide substantial evidence for the construct validity of the FCAT test as a measure of reading comprehension in which the proportion of items that assess "higher order thinking skills" increases substantially from third through tenth grade. (Contains 6 tables and 3 figures.) [Preliminary report was prepared December, 2004, by the Florida Center for Reading Research.]