Serious games foster the acquisition of complex problem-solving skills. Assessment of such skills should be in line with instruction, and within a serious game environment its content validity should equal face-to-face assessment. Research on assessment in serious gaming has remained rather scarce. This article shows how assessment can be implemented in serious gaming in a way that assures content validity. The core of the authors' validation method entails mapping learning activities (as contained in the game scenario) on performance indicators and outputs (as derived from formal attainment levels). They present how they have elaborated and applied the method for an assessment game for ICT managers in secondary vocational education. They describe the procedure and extent to which this assessment is content-valid compared to face-to-face assessment.