Increased physical activity (PA) has been associated with a decreased risk for the occurrence and recurrence of many cancers. PA is an important outcome measure in rehabilitation interventions within cancer and may be used as a proxy measure of recovery or deterioration in health status following treatment and in the palliative care setting. PA is a complex multi-dimensional construct which is challenging to measure accurately. Factors such as technical precision and feasibility influence the choice of PA measurement tool. Laboratory-based methods are precise and mainly used for validation purposes, but their clinical applicability is limited. Self-report methods such as questionnaires are widely used due to their simplicity and reasonable cost; however, accuracy can be questionable. Objective methods such as pedometers measure step count but do not measure intensity, frequency or duration of activity. Accelerometers can measure PA behaviour at both ends of the movement spectrum from sedentary to vigorous levels of activity and can also provide objective data about the frequency, intensity, type and duration of PA. Balancing precision with ease of use, accelerometry may be the best measure of PA in cancer-based studies, but only a small number of studies have incorporated this measurement. This review will provide a background to PA and an overview of accelerometer measurement as well as technical and practical considerations, so this useful tool could be more widely incorporated into clinical trial research within cancer.