Learning in games has historically been assessed indirectly and/or in a post hoc manner. What´s needed instead is real-time assessment and support of learning based on the dynamic needs of players. We need to be able to experimentally determine the degree to which games can support learning, and how and why they achieve this objective. In this chapter we describe an approach to designing and developing evidence-based diagnostic assessments that may be embedded in a game environment. When embedded assessments are so seamlessly woven into the game that they´re virtually invisible, we call this 'stealth assessment.” Embedding assessments within games provides a way to monitor a player´s current level on valued competencies, and then use that information as the basis for support, such as adjusting the difficulty level of challenges or providing a report for the teacher.