Physical activity (PA) is important for young children in achieving positive health outcomes and there is growing evidence suggesting that PA and fitness play an important role in children’s brain development and academic performance. Studies of preschool children typically show that few meet recommended daily PA levels. Meanwhile many preschoolers worldwide attend preschools, but little is known about their on-campus PA or the factors that influence it, especially during classroom sessions. The purpose of this study was to use a validated instrument to conduct a micro-analysis of student PA levels, lesson contexts, and teacher prompts for PA during classroom sessions. We assessed these factors during 165 classroom sessions taught by 25 teachers in four preschools. The sessions occurred prior to (PRE-PE) or following the PE lessons (POST-PE) that we described in an earlier paper (Chow et al., 2015) . Children spent classroom time primarily sitting (76%) and standing (13%), while engaging in PA (mostly of low or moderate intensity) 11% of the time. PA occurred mostly during lesson transitions and music time. Overall, boys were more active than girls (MVPA% = 12.1 vs. 8.8), and teachers rarely promoted either in-class or out-of class student PA. The study contributes to the limited literature on PA and its related contexts in preschools. The results have implications for providing pre-service and in-service teacher education related to the importance of PA and how to implement it. Expanded surveillance studies are recommended. In addition to using questionnaires and accelerometers or pedometers to assess PA, the studies should include observation methods so that important on-site and potentially modifiable contexts can be assessed directly. Copyright © 2015 by authors and Scientific Research Publishing Inc.