Child passenger safety remains an important public health problem because motor vehicle crashes are the leading cause of death for children, and the majority of children ride improperly restrained. Using a mobile app to communicate with parents about injury prevention offers promise but little information is available on how to create such a tool. The purpose of this article is to illustrate a theory-based approach to developing a tailored, smartphone app for communicating child passenger safety information to parents. The theoretical basis for the tailoring is the elaboration likelihood model, and we utilized the precaution adoption process model (PAPM) to reflect the stage-based nature of behavior change. We created assessment items (written at =6th grade reading level) to determine the child's proper type of car seat, the parent's PAPM stage and beliefs on selected constructs designed to facilitate stage movement according to the theory. A message library and template were created to provide a uniform structure for the tailored feedback. We demonstrate how messages derived in this way can be delivered through new m-health technology and conclude with recommendations for the utility of the methods used here for other m-health, patient education interventions.