This study describes the development and validation of a multidimensional measure of preadolescent and adolescent readers' abilities to apply reading comprehension strategies necessary for understanding challenging academic texts. The Strategy Use Measure (SUM) was designed with the intention of being pedagogically informative to the increasingly multilingual student population in the U.S. in grades 6 through 8. The SUM aims to measure four areas of knowledge and skill that are widely purported to support the use of reading strategies: (a) morphological awareness, (b) knowledge of cognates, (c) ability to relate micro- and macro- ideas within a text, and (d) the ability to use intra- and inter-sentential context clues for defining unfamiliar words. The test was developed following a principled, iterative process to instrument development, employing Rasch models and qualitative investigations to test hypotheses related to the instrument's validity. Findings suggest promising evidence for the validity and fairness of this multidimensional measure.