Traditional device-based physical activity measurement methods are unable to accurately differentiate between low intensity movements (i.e., lying/sitting/reclining/standing/non-wear). These limitations prevent the development of accurate 24-hour behavioural profiles. This is important as many countries have now adopted 24-hour movement guidelines, combining physical activity, sedentary behaviour, and sleep. Classification of activity behaviours using raw accelerometer data is becoming more prominent and is consistently acknowledged as an avenue of progression. This presentation will explore the development and utility of a dual-accelerometer system for capturing 24-hour movement patterns in children and adults. A series of studies will be presented, starting in a structured laboratory environment before moving into free-living settings. The application of these measures in several study populations (including a national child cohort and adult shift workers) will be demonstrated, before ending with a summary of the challenges and future research directions of this field.