This article describes the Wellness Enhancing Physical Activity in Young Children (WE PLAY) teacher training, which was designed to assist early childhood educators to promote physical activity among preschoolers in child care. We describe the WE PLAY intervention and its grounding in constructs from theories of health behavior and an implementation science framework. Fidelity, feasibility, and acceptability data from the WE PLAY pilot study, a cluster randomized controlled trial (RCT) with six Head Start programs in Massachusetts. Data, collected between October 2017–May 2018, are from teachers and supervisors at 3 preschool programs who participated in the WE PLAY (intervention) group. To understand program feasibility and acceptability, we used the Usage Rating Profile-Intervention (URP-I; n = 13) and key informant interviews (n = 5). The URP-I is a validated teacher survey with 6 subscales (Acceptability, Understanding, Feasibility, Family-School Collaboration, Systems Climate, and Systems Support). It was administered twice; immediately after users completed the first component, an online training (week 2), and after implementation of all program components (week 4). WE PLAY was implemented as it was intended, and it was considered acceptable and feasible to users. There was an increase in users' understanding of how to implement the program between weeks 2 and 4, and a concomitant decrease in the amount of additional systems-level supports users thought they would need to implement WE PLAY between weeks 2 and 4. WE PLAY was easily understandable and feasible to implement in real world settings, it was highly acceptable to users, and it deserves further testing. (author abstract)