Peers have an important influence on the physical activity of youngsters, because most physical activities require other peers as play partners. Experimental studies have corroborated this idea, while using artificial settings in which children had only one other interaction partner. However, youth’s activities and the number and type of interacting partners vary heavily throughout the day. Therefore, unobtrusive and continuous measures of physical activity and peer proximity are preferred to assess the full scope of the influence of peer interaction on physical activity. This study has 3 research aims: 1) investigating the overall effect of peer presence, 2) examining the influence of group size, and 3) assessing the effect of interaction frequency with peers on physical activity. This is done by handing out the Wearable Lab to 1,200 youngsters (9-15 years old): a smartphone with a custom-built research app and a wearable pedometer (Fitbit). Participants use the Wearable Lab for three separate weeks over a six month period (January 2016-June 2016). Physical activity is measured by the pedometer (steps and minutes MVPA) and GPS coordinates (cycling activities) and averaged to activity per 15 minutes. Peer proximity is measured by the research smartphones scanning for other participants’ phones that are within Bluetooth range (approximately 10 meters) every 15 minutes. Regression analyses and social network modeling will test 1) the effect of peer presence by comparing physical activity levels in a group setting compared to an individual setting, 2) the effect of group size by examining whether youth become more physically active when the group size of interacting peers increases and 3) the effect of interaction frequency with peers by analyzing whether participants are more physically active when they interact with peers whom they see more frequently, than peers whom they see less often.