“eMoodie” is a developmentally-informed application specifically designed for experience sampling (ESM) and ecological momentary assessment (EMA) research with children and adolescents. It was created as a research tool for the express purpose of studying emotion and socialization patterns, digital technology use, and health factors (e.g., physical activity and sleep) in young people. The app delivers both recurring surveys using notifications and standard questionnaires. It incorporates interactive feedback mechanisms using points and achievements to help engage participants. In keeping with the rich history of using pictures and vignettes to better communicate the meaning of concepts in studies with developmental samples, the app enables researchers to specify graphical elements into question and answer formats. Another unique feature of the app is its ability to operationalize variables of interest using unobtrusive, automatically-logged data that can be collected by the app from sensors and from usage statistics on the smartphone or tablet where it is installed. For example, data from the accelerometer can provide an index of activity beyond self-reported levels. All specifications for a research study can we programmed using a separate web-interface. eMoodie’s user interface and graphics were designed to be both child-friendly and with a modern aesthetic so as to be appealing to older adolescents and young adults at the same time. The flexibility of this application to collect different types of objective, real-time data in conjunction with ecologically-valid ESM data will contribute to advancing the explanatory power of existing experience sampling methodologies used in developmental research.