Assessment of specific learning disabilities (SLD) in educational settings is one important function of a school psychologist. The federal definition of SLD describes "underlying cognitive processing deficits" as part of the assessment criteria, that is also incorporated in several states' SLD eligibility criteria, that is difficult to directly evaluate using traditional assessment methods. Traditionally, use of norm-referenced scores have been used to compare an examinee's relative performance on assessment tests. Such a method does not provide direct evidence of underlying cognitive processing deficits associated with poor academic achievement. In an effort to directly inform this cognitive processing ability and academic proficiency relation, the current study utilized measures of absolute level of performance--"W" scores from cognitive (COG) and achievement (ACH) tests from the Woodcock-Johnson IV (WJ IV)--to search for optimal cognitive predictors of proficiency in reading and mathematics domains. The study included two phases. The first phase consisted of determining minimum proficiency standards for WJ IV ACH tests related to reading and mathematics SLD eligibility areas through expert review of test item content to establish cut scores for proficient performance. The second phase employed theoretically relevant WJ IV COG tests as predictor variables in multiple receiver operating characteristic (ROC) curve analyses for proficiency classification purposes for the reading and mathematics SLD eligibility area clusters and their contributing WJ IV ACH tests. Results indicate that the WJ IV COG tests are generally good to excellent predictors of academic proficiency, when considering absolute levels of performance. Oral Vocabulary, a measure of Comprehension-Knowledge ("Gc"), and Number Series, a measure of Fluid Reasoning ("Gf"), demonstrated the greatest diagnostic accuracy for classification of proficiency in reading and mathematics. Groundwork has been laid for establishing cognitive predictors that accurately predict proficiency in reading and mathematics that can strengthen the case that an examinee meets the underlying criteria for an SLD (i.e., a related cognitive processing deficit). [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]