The recent immense diffusion of smartphones has significantly upgraded the role of mobile user interfaces in interventions that build and/or maintain healthier lifestyles. Indeed, high-quality, user-centered smartphone applications are able to serve as advanced front-ends to such interventions. These smartphone applications, coupled with portable or wearable sensors, are being employed for monitoring day to day health-related behaviors, including eating and physical activity. Some of them take one step forward by identifying unhealthy behaviors and contributing towards their modification. This work presents the design as well as the preliminary implementation of the mobile user interface of SPLENDID, a novel, sensor-oriented intervention for preventing obesity and eating disorders in young populations. This is implemented by means of an Android application, which is able to monitor the eating and physical activity behaviors of young individuals at risk for obesity and/or eating disorders, subsequently guiding them towards the modification of those behaviors that put them at risk. Behavior monitoring is based on multiple data provided by a set of communicating sensors and self-reported information, while guidance is facilitated through a feedback/encouragement provision and goal setting mechanism.