Engagement is a worthwhile psychological construct to examine in the context of online training and video games. In this context, previous research suggests that the more engaged a person is, the more likely they are to experience overall positive affect while performing at a high level. This research builds on theories of engagement, Flow Theory, and Cognitive Load Theory, to operationalize engagement in terms of cognitive load, affect, and performance. Studies One and Two examined the development of a puzzle-based video game as a platform for measuring engagement. Results of these studies demonstrated that: cognitive load could be measured using a secondary task, affect could be measured using a novel game-clock mechanism, and performance could be captured incrementally throughout gameplay. Study Three compared traditional linear gameplay, choice-based gameplay, and adaptive gameplay that used a real-time measure of engagement to select game levels. Results showed that those in the Adaptive condition performed higher compared to those in the other two conditions without any degradation of overall affect or self-report of engagement. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]