The achievement gains of California students with disabilities (SWDs) on the California Standards Test (CST)-English Language Arts have surpassed the achievement gains of students with no reported disabilities in recent years, and so the special education achievement gap seemed to be closing. However, the reported achievement for SWDs has not factored in changes in the tested student populations that resulted from the introduction of modified assessments, suggesting that the achievement gains of SWDs are misleading. In this descriptive study I identified population characteristics of California students taking the various assessments and described how achievement trends were influenced as alternative assessment tests were implemented and as the numbers of students taking the various Standardized Testing and Reporting (STAR) tests fluctuated. Using published data from the California Department of Education STAR and DataQuest systems, I developed scenarios to evaluate the achievement of SWDs by exploring how the reported results were affected by changes in tested populations. I found that the achievement of all SWDs has improved only marginally and that the achievement gap has continued to increase, contradicting reports that the gap is closing. I also examined whether tests have been administered to appropriate students, according to NCLB regulations, and identified weaknesses in the federal regulations and state guidelines associated with assessing SWDs. I anticipate that these findings will provide policymakers, administrators, and special educators with a better assessment of the progress made by the State of California toward closing the special education achievement gap and of the consequences of current regulations, guidelines, and practices. [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.]