Patient-reported outcomes (PROs) offer additional insight into a patient's daily activity and have been shown to have a prognostic and therapeutic value. However, as with all self-reported outcomes, many PROs are associated with response and recall biases. Given the prognostic importance of performance status and PROs in addition to their impact on treatment decisions, there is a need for feasible, real-time, objective collection of a patient's daily activity. Recent advances in wearable technology have provided oncologists with new opportunities to obtain real-time, objective physical activity data. The integration of wearable activity monitors into children's cancer care will help increase our understanding of the associations between physical activity and the prevention and management of the disease, in addition to other important cancer outcomes. Challenges to be addressed in the use of wearable technologies include lack of standardization in the types of monitors to be used and how their data are being collected, analyzed, and interpreted.