Human computation is a field that studies the employment of humans' computing ability to solve complex problems that computers cannot easily solve in time. Games present one platform for human computation in an entertaining manner. Pioneered by Luis Von Ahn and his colleagues, many games with a purpose (GWAP) systems have been developed in recent years, such as ESP, GWAP for the Semantic Web, and CityExplorer. Computational tasks, including image tagging, building the Semantic Web, and collecting geospatial data for cities, are “crowdsourced” to the players and solved by human power.