Many students with disabilities need to use accommodations to meaningfully access instruction and to be appropriately assessed. During the past decade, the use of assessment accommodations has dramatically increased the participation rates of students with disabilities in statewide testing. Assessment accommodations can be defined as "changes in materials used for testing" (Thurlow, Elliott, & Ysseldyke, 2003, p. 30). The purpose of this topical review is to analyze how accommodations are used by students with disabilities on statewide tests, and their implications for accountability systems. This topical review also analyzes the state accommodations policies in four core states in which the Educational Policy Reform Research Institute (EPRRI) is conducting research (California, Maryland, New York, and Texas) to illustrate a range of accommodation policies. Participation and Accommodation Definitions is appended. (Contains 13 tables and 1 figure.)