People play computer games – to explore simulated worlds, combat foes and overcome challenges – the computer software monitors their progress. It continually collects data about players’ actions, making inferences about their goals and strategies to set appropriate new challenges. The term was first used by Valerie Shute in 2005 to describe the automated assessment process in a system named Smithtown to teach principles of microeconomics. Students explored the Smithtown simulated world and altered variables, such as the price of coffee and the incomes of inhabitants. The pedagogy that underlies stealth assessment is competency learning. The teacher estimates what the student knows and can do, continually providing tasks and assessment that are matched to the student’s competency. Stealth assessment works best when the assessment strategies, the game and the simulated world are all developed together through a process of evidence-centred design that applies not only to the assessment but also the gameplay.