CyGaMEs invites youth ages nine and older to play its award-winning, online, instructional videogame Selene to learn the Solar System’s basic geological processes: “Blast away at what will quickly become a full-fledged, pockmarked moon like our own. Replicate the Moon’s 4.5-billion-year history. Follow with our MoonGazers hands-on activities.” Prepared with new knowledge that makes Moon viewing meaningful, Selene players go outside to explore the Moon from their own backyards. CyGaMEs, a principled approach to instructional game design and embedded assessment, applies cognitive science analogical reasoning theory to translate a to-be-learned conceptual domain into an instructional game world’s relational structure, gameplay, and game goals. Rigorous specification maps relational structure from targeted STEM content to game world. CyGaMEs is used in formal, informal, and non-formal contexts. CyGaMEs Selene causes and measures learning as players discover and apply STEM concepts. CyGaMEs’ embedded assessment “timed report” collects data every 10 seconds, measuring learning trajectories and identifying learning moments. CyGaMEs’ flowometer measures affect, supporting trace of perceived experience and investigation of interplay between learning and affect. CyGaMEs supportsevaluation and assessment. CyGaMEs offers online research environments designed for experimental control and random assignment. It provides rigorous empirical evidence; supports robust, strategic research of causal claims disentangled from student selection; and enhances effective teaching, game-based cyberlearning, and embedded assessment. CyGaMEs also offers professional development. Aptly designed videogame worlds provide common experiences that prepare educators and learners to achieve success. CyGaMEs enhances control over what games do: engage learning through doing, discovery, and inquiry.