This study investigated the application of multidimensional item response theory (IRT) models to validate test structure and dimensionality. Multiple content areas or domains within a single subject often exist in large-scale achievement tests. Such areas or domains may cause multidimensionality or local item dependence, which both violate the assumptions of the unidimensional IRT models currently used in many statewide large-scale assessments. An empirical K-12 science assessment was used as an example of dimensionality validation using multidimensional IRT models. The unidimensional IRT model was also included as the most commonly used model in current practice. The procedures illustrated in this real example can be utilized to validate the test dimensionality for any testing program once item response data are collected. (Contains 7 tables and 4 figures.)