This study provides empirical evidence of a highly specific use of games in education--the assessment of the learner. Linear regressions were used to examine the predictive and convergent validity of a math game as assessment of mathematical understanding. Results indicate that prior knowledge significantly predicts game performance. Results also indicate that game performance significantly predicts posttest scores, even when controlling for prior knowledge. These results provide evidence that game performance taps into mathematical understanding. (Contains 2 figures.)