a b s t r a c tThis research provides a critical consideration of the outcome measures used to assess physical activity in individuals with severe mental illness. A narrative synthesis was utilised to provide a simple juxtapose of the current research. A sensitive topic-based search strategy was conducted in order to identify studies that met the eligibility criteria. Fifty two studies met the inclusion criteria and 5 were identified specially as validation studies. The current research identified several methodological shortcomings. The justification and choice of outcome measure used is often weak and only five studies have validated a specific outcome measure of physical activity. Within these validation studies, the validation process often lacked a consideration of agreement between measures. Accelerometers have been most frequently used as a criterion measure, notably the RT3 tri-axial accelerometer. Objective based measures may be best placed to consider physical activity levels, although, methodological considerations for the utilization of such tools is required. Self-report questionnaires have benefits for use in this population but require further validation. Researchers and clinicians need to carefully consider what outcome measure they are using and be aware of the development, scope and purpose of that measure.Crown