The Primary Goal is to conduct a 5-10 week randomized controlled trial (RCT) of the Fitbit Flex, a popular, affordable, wearable physical activity tracking device, and the Fitbit mHealth app. The target population will be childhood cancer survivors 14-18 years old recruited from Seattle Children's Hospital. For the intervention group, peer influences will be engaged via a private, social network (e.g. a Facebook group) customized for survivors. Measurements will be completed (1) at baseline, prior to randomization, and (2) during the final week of the intervention period (follow-up measure). This study will provide initial proof of concept and allow for further customization of the intervention for childhood cancer survivors in anticipation of a future, larger proposal to study physical activity and related outcomes over a multi-year period.

Inclusion Criteria:

• in remission for childhood cancer and previously treated for childhood cancer at Seattle Children's Hospital
• ≥1-year post-cancer therapy
• ambulatory without any known medical contraindications to participation
• able to complete surveys in English
• agree to install and share data from the FitBit smart phone app with the investigators

Exclusion Criteria:

• currently or previously used a wearable physical activity sensing device to improve physical activity