Comparability refers to the commonality of score meaning across testing conditions including delivery modes, computer platforms, and scoring presentation. As state education agencies introduce online tests and online scoring of constructed-response questions, score comparability becomes important. For example, it should be a matter of indifference to the examinee whether the test is administered on computer or paper, or whether it is taken on a large-screen display or a small one. To the extent that test results are invariant across such conditions, scores may be used interchangeably. This paper explores the comparability issues related to online assessment at the elementary and secondary level, and reviews the available research.