Objective: To explore young people's perceptions of pedometers and investigate behaviours exhibited while being monitored. Design: Qualitative design using six focus groups with participants (mean age 14.7 years). Setting: Study participants (n = 24) were randomly selected from a previous study of 123 young people aged 14-15 years from three secondary schools in New South Wales (NSW), Australia. Methods: Participants wore pedometers (Yamax CW700) and accelerometers (ActiGraph GT3X+) simultaneously for 7 days. Accelerometer output was used to categorise participants into one of six focus groups (three boys groups and three girls groups): (1) low active (<30 minutes moderate to vigorous physical activity [MVPA]/day), (2) medium active (30-60 minutes MVPA/day), and (3) high active (=60 minutes MVPA/day). Participants were questioned on their perceptions of the monitoring process and the behaviours that they exhibited while wearing pedometers. A hybrid approach to data analysis identified key concepts, which were thematically analysed. Results: The two main themes identified were (1) participants' perceptions of the monitoring process and (2) behaviour exhibited while being monitored. Overall, participants' attitudes towards objective monitoring were positive. A large proportion reported changing their levels of physical activity during the monitoring process, and 87.5% of focus group participants reported shaking their pedometers to increase their step counts. The medium and high active groups reported changing their activity patterns more than the low active groups. Conclusion: Our findings are consistent with previous quantitative studies that suggest reactivity and tampering are commonplace among young people. Pedometers may have more utility as an intervention strategy for increasing activity rather than as a method for assessing habitual activity levels.