Attuning a mobile simulation game for school children using a design--based research approach AbstractWe report on a design--based research study that was conducted over nine months. It chronicles the development and implementation of HeartRun, a cardiopulmonary resuscitation (CPR) training approach for school children. Comparable to an unexpected emergency, HeartRun consists of authentic activities involving different roles, game tasks, locations and physical objects to support process--oriented learning for first responders. It aims to enhance the psychological preparedness of the rescuer and thus promotes a more prompt and appropriate response. In this paper, we describe a cycle of three design--based research (DBR) studies in which HeartRun was explored with school children. In order to better understand how to design mobile game environments that support dimensions of seamless learning, we analysed children and their knowledge--building practices while learning with HeartRun. The mobile game has evolved significantly from its initial conception through an iterative process of (re) designing and testing the synchronization between physical and digital worlds, learner collaboration and ubiquitous knowledge access, i.e. dimensions of mobile seamless learning activities. Based on our experiences, we conclude by discussing challenges and shortcomings of mobile game-based learning environments.