In the arena of educational testing, accessibility refers to the degree to which students are given the opportunity to participate in and engage a test. Accessibility theory is a model for examining the interactions between the test-taker and the test itself and defining how they may decrease some students' access to the test event, ultimately reducing the validity of subsequent test score inferences. The paper describes the application of accessibility theory to the development of tests that are optimally accessible for all students, with particular regard to students identified with disabilities. The author uses example test items to describe these sources of error and discusses how they may be addressed during the design phase. (Contains 4 figures and 1 table.)