Games can provide insights into children's thinking and concepts related to mathematics. Friederike Heinz develops and tests communication-intensive learning games to capture concepts of numbers, quantities, and operations. Using video analyses of game situations in 2nd and 3rd grade, she examines the potential of such games as a tool for informal initial diagnosis. The author discusses diagnostic insights into the learning status and challenges faced by the participants and provides an overview of the application possibilities of diagnostic games in the classroom.