Objectives: To explore children's perceptions of the factors influencing their engagement in physical activity during the "critical" lunchtime period, using a social-ecological framework. Design: This study was an in-depth descriptive qualitative design. Methods: Fifty-four South Australian children aged 10-13 years participated in same-gender focus groups. Transcripts, field notes and activity documents were analysed using content analysis. Using an inductive thematic approach, data were coded and categorised into perceived barriers and facilitators according to a social-ecological model. Results: Children identified a range of environmental, social and intrapersonal barriers and facilitators. Bullying/teasing, the school uniform and school rules were exposed as explicit barriers to lunchtime play. Other important barriers included lack of access to, and poor suitability of, space, lack of access to programs/facilities and equipment, and lack of peer and teacher support. Perceived facilitators of lunchtime physical activity centred on access to equipment, enjoyment, motivation to improve skills, and peer support and acceptance. The freedom to make up or modify rules for games was also perceived to be a facilitator of lunchtime play. Conclusions: Communicating with children has been an effective approach in uncovering perceived barriers and facilitators to lunchtime play that may not have been previously considered in the quantitative correlate literature. Lunchtime interventions targeting children's physical activity should focus on addressing the barriers perceived to be important to lunchtime play.