The five papers in this special issue of the "Journal of Applied Testing Technology" address fundamental issues of validity when tests are modified or accommodations are provided to English Language Learners (ELL) or students with disabilities. Three papers employed differential item functioning (DIF) and factor analysis and found the underlying constructs measured by tests do not change among these groups of students. Despite this strong finding, consistent and large score differences are present across groups. Such consistent and large score differentials among these groups on cognitive ability tests would be ideally contrasted with findings from alternative measures (e.g., portfolio's, performance assessments, and teachers' ratings). Two papers examine current methods used to identify and classify both ELL and students with disabilities, while other papers examine the performance of students with specific disabilities (e.g., deaf, mental retardation). The impact of modifications and accommodations on score comparability is discussed in relation to professional standards and current validity theory. (Contains 2 footnotes.)