Increasing the physical activity of typically sedentary adult populations is at the forefront of the public health agenda. This review addresses the challenges in defining and measuring physical activity in this target group, for a number of purposes, namely, scientific or academic inquiry, surveillance, clinical application and programme evaluation. First, we clarify the conceptual distinctions between the terms sedentarism, physical inactivity, physical activity and energy expenditure. Next, we review and compare the utility of different approaches for quantifying and expressing physical activity in these populations. Physical activity in typically sedentary populations is most likely a simple pattern of behaviour that has been largely obscured by existing measures and its expression as energy expenditure. Existing self-report methods are practical, but suffer from floor effects and recall bias. Walking, the most important activity to assess in this target group, is very difficult to measure through self-report methods. Motion sensors are more appropriate for quantifying physical activity behaviours in typically sedentary populations. Of the 2 types of motion sensors - the accelerometer and the pedometers--the latter is more appealing because it is both an affordable and a 'good enough' measure of physical activity, specifically ambulatory activity. Although a common measurement approach would greatly facilitate our understanding of physical activity behaviour patterns, the selection of an approach ultimately depends on the purpose of the study and to a great extent, its budget. Researchers, clinicians and practitioners interested in accurately capturing the lower end of the continuum of physical activity (that is characteristic of sedentary populations) must thoughtfully consider the relative advantages and disadvantages of the available approaches.