Accurate assessment of physical activity energy expenditure is essential to (1) quantify the relationships between energy expenditure and children’s health outcomes and (2) assess intervention/prevention effects. However, energy expenditure is difficult to assess in free-living conditions. A combination of heart rate (HR) and accelerometry data provides a more precise estimate of energy expenditure than either HR or accelerometry alone, when compared to a criterion measure of direct or indirect calorimetry. However, very few available devices exist which capture both HR and accelerometry. Although some scientific- grade (e.g., ActiHeart) and consumer-grade wearable devices (e.g., Fitbit) incorporate multiple sensors (i.e. accelerometry and HR via chest leads or photoplethysmography), restrictions to raw signal data (FitBit) or the need to change chest leads daily (ActiHeart) makes it problematic to use such devices to measure energy expenditure in children. Moreover, these devices were not developed with children in mind, and therefore, have limitations regarding size, comfort, and the need for daily care/maintenance. The current study leverages microcontroller wearable electronics’ technology to develop a small device that adheres to the human body (referred to here as the ‘PATCH’). The PATCH is the size of a postage stamp and incorporates multiple sensors (i.e. accelerometry and photoplethysmography) in a single device. Using multiple channels of raw signal data, we can more accurately estimate energy expenditure under free-living conditions. The PATCH is far less obtrusive than existing scientific grade integrated devices (i.e., ActiHeart) or commercial wearable devices (i.e., FitBit), and thus has the potential to improve acceptability and wear-time, particularly among children. Our long-term goal is to advance the measurement field for epidemiologic- and intervention- based studies that focus on measuring 24-hour movement and energy expenditure. The objectives of the proposed project are to conduct a series of studies that include both laboratory and field- based protocols to establish the preliminary reliability, validity and utility/feasibility of the new PATCH technology for measuring children’s energy expenditure in free-living conditions. We will evaluate the validity of the PATCH to measure physical activity and HR of children (age 3-8 years old) compared to criterion measures of accelerometry and electrocardiography, respectively. We will develop equations to predict energy expenditure and examine initial concordance compared to indirect calorimetry. Using quantitative and qualitative methodology, we will also assess the feasibility and acceptability of wearing the PATCH over extended periods of monitoring wear time (e.g., 30 days). This technology has the potential to become a research standard for intervention and epidemiological studies of energy expenditure among children. Results from the current study will inform the next phase ...