Recently there has been a push to consider assessment, and specifically formative assessment, in conjunction with game-based learning. Formative assessment is meant to provide feedback to students that can guide their learning process. Central to the production of effective formative assessments are the concepts of validity and reliability. Validity refers to the extent to which assessment scores indicate the amount that a learners has of an underlying construct, defined as an unobservable trait. This chapter focuses on the first stages of the process of formative assessment creation—the specification of constructs underlying learning goals. Constructs are specified by the process of theory specification, construct generation, test item development, and construct refinement. By considering construct validity in assessment development, game developers can collect needed data to optimize games and provide evidence of learning.