The impetus for providing a large array of test accommodations is to ensure that all students with disabilities can meaningfully participate in state and district assessment systems. The range of accommodation options varies across states, as do the characteristics of students receiving accommodations. Data from several states now indicate that about 50% of those participating in general assessments do so with accommodations (Bielinski, Ysseldyke, Bolt, Friedebach, & Friedebach, this issue; Elliott, Bielinski, Thurlow, DeVito, & Hedlund, 1999; Trimble, 1998). However, there is other evidence of considerable variability in rates (Thompson & Thurlow, this issue). Data from states that track accommodation use indicate that the majority of students receiving accommodations are students with learning disabilities. This is not surprising, since roughly 50% of students identified with a disability are classified as having a learning disability. Since test results are to be disaggregated for students with disabilities, many of whom use accommodations, it is important to demonstrate that scores obtained from accommodated administrations accurately reflect achievement.