The amount and quality of infants? interaction with caregivers impact their opportunities for optimal development, providing the foundation for lifelong health outcomes. Nevertheless, around the world, some 250 million young children are at risk of not achieving their developmental potential. To improve development for these at-risk children, evidence-based approaches include supporting caregivers to provide nurturing care. However, scaling these services and support for caregivers is challenging in many low-resource delivery environments, where over-taxing of frontline healthcare workers is a limiting constraint. For infant development, mHealth technologies have the potential to solve this problem by providing tailored content directly to caregivers, involving and empowering them to promote infant development, promoting and facilitating interactions with health workers when areas of concern are identified and, therefore, and expanding the reach of healthcare systems. The objectives of this project are to develop mHealth smartphone technology which can be used to engage primary caregivers directly in the active monitoring of their infants? development, and to provide tailored feedback and support for the provision of nurturing care. In addition, we will also prospectively assess the implementation characteristics of the technology?usability, acceptability, and sustainability?for caregivers and health workers. Aim 1 is to use an agile design approach to develop and audience test a smartphone application to engage caregivers in monitoring their infants? development and to provide tailored anticipatory guidance for nurturing care. Aim 2 is to assess the implementation characteristics of the smartphone application through a longitudinal, six-month usability trial, to determine caregiver engagement over time and to assess the perceived usefulness of the application. Aim 3 is to determine the effectiveness of a smartphone-based, real-time caregiver feedback intervention to promote positive infant developmental outcomes and improved caregiving behaviors. This exploratory/developmental research application responds to PAR 19-376?s call to ?study the development, validation, feasibility, and effectiveness of innovative mobile health (mHealth) interventions or tools specifically suited for low- and middle-income countries (LMICs)?, in this case a culturally-based, family-focused approached to engaging caregivers in nurturing care for infants at-risk. This is a multidisciplinary proposal, involving pediatrics, developmental and community psychology, physical therapy, and biomedical engineering. Our results will contribute to the evidence base for the use of mobile technology (client-focused applications providing real- time interactive feedback) to directly engage target populations around important public health priorities while building research capacity for mHealth in an LMIC, Guatemala. This application is a partnership between the Children?s Hospital Los Angeles (a U.S. institution), Emory University (a U.S. institution), and Maya Health Alliance (an LMIC institution). The proposed plan will strengthen the mHealth research capabilities at the LMIC institution and in Guatemala through the development of research and programming personnel and resources.