The purpose of this study was to determine the constructs measured by the WISC-IV and the consistency of measurement across large normative and clinical samples. Competing higher order four- and five-factor models were analyzed using the WISC-IV normative sample and clinical subjects. The four-factor solution is the model published with the test manual. In the five-factor model, the POI differentiated into a visual-spatial factor (consisting of Block Design and Picture Completion) and a fluid reasoning factor (consisting of Matrix Reasoning and Picture Concepts, with and Arithmetic). The five-factor solution included Inductive Reasoning (IR), consisting of Matrix Reasoning and Picture Concepts, as a narrow ability subsumed under the FRI (Gf). When all 15 WISC-IV subtests were considered, both four- and five-factor models were suitable and showed close model-data fit. Further, both models generally demonstrated full factorial invariance between clinical and nonclinical samples. Interpretation of the fifth factor is discussed. (Contains 6 tables, 2 figures, and 3 notes.)