This paper reviews 15 research studies that: (1) examined effects of particular accommodations or groups of accommodations on performance: and (2) employed experimental and quasi-experimental research designs that allowed examination of the effect of the accommodation(s) on English Language Learners (ELLs) and non-ELLs. Studies looked at one or more of the following types of accommodations: (1) linguistic simplification; (2) customized English dictionaries and glossaries; (3) use of the native language; (4) reading items and/or directions aloud; and (5) providing extra time in combination with other accommodations. Although research on accommodations for ELLs is inconclusive, two kinds of accommodations appear to hold promise: native language versions of assessments and linguistic simplification of English versions. Combining specific direct linguistic support accommodations with specific indirect linguistic support accommodations also appears to support ELL performance on assessments. The paper reviews states' SY 2000-2001 policies related to testing accommodations for ELLs. Policies were often organized explicitly around the needs of two student groups: ELLs and students with disabilities (SDs). Overall, the most noticeable trend with regard to treatment of content areas is that accommodations providing direct linguistic support to ELLs were more likely to be prohibited for English language arts than for other content areas. A majority of states arranged accommodations within a taxonomy developed for students with disabilities (timing/scheduling, setting, presentation, and response). Recommendations for selecting appropriate accommodations for NAEP include the following: (1) Use an ELL-responsive framework as a tool for selecting appropriate accommodations for ELLs; (2) Use accommodations that are responsive to ELLs to provide direct or indirect linguistic support; (3) Use student background variables to inform selection of appropriate accommodations based on a consistent operational definition of English language learner, the student's level of English language proficiency, and the language of instruction; and (4) Use accommodations supported by research. In addition, the paper recommends that a panel of experts be charged with identifying at least two accommodations to be field tested for use on the National Assessment of Educational Progress (NAEP). Three appendixes include: (1) Overview of Accommodations Research; (2) Data from States' Assessment Policies for SY 2000-2001; and (3) NAEP Accommodations. (Contains 8 footnotes, 4 figures, and 19 tables.) [This paper was commissioned by the National Assessment Governing Board (NAGB) to serve as background information for conference attendees at the NAGB Conference on Increasing the Participation of SD and LEP Students in NAEP. The data presented in this paper are excerpted from a project conducted for the US Department of Education, Office of English Language Acquisition.]