The Special Issue of Human-Computer Interaction, 2015 is o devoted to papers on game-related HCI research at CHI. Iacovides, Cox, McAndrew, Aczel, and Scanlon take a close look at the connection between player involvement and learning. Alankus and Kelleher created a motion-based game to improve stroke survivors' rehabilitation when this is unsupervised by a clinician. In particular, their focus is on reducing the compensatory motions that patients tend to make when doing unsupervised exercises. Sim, Read, Gregory and Xu explore whether children can be guided through a participatory design process in designing serious games for children in other cultural contexts. Deterding presents his 'lens of intrinsic skill atoms', a framework for understanding and engaging in what he terms gameful design, more broadly known as gamification. Isbister and Mueller's work targets games that use movement sensing as input, something that all major gaming consoles now offer in addition to buttons and joysticks, and that is incorporated into many modern mobile and tablet platforms.