An achievement test score can be viewed as a joint function of skill and will, of knowledge and motivation. However, when interpreting and using test scores, the "will" part is not always acknowledged and scores are mostly interpreted and used as pure measures of student knowledge. This paper argues that students' motivation to do their best on the assessment--their test-taking motivation--is important to consider from an assessment validity perspective. This is true not least in assessment contexts where the assessment outcome has no consequences for the test-taker. The paper further argues that the quality of assessment of test-taking motivation also needs attention. Theoretical and methodological issues related to the assessment of test-taking motivation are presented from a validity perspective, and findings from empirical studies on the relation between test stakes, test-taking motivation and test performance are presented. (Contains 1 figure.)