Purpose: Physical recovery from critical illness is complicated by neuromuscular weakness. Evidence suggests mobility commencing within the intensive care unit results in improved function upon discharge. Despite this, persistent inactivity is reported throughout hospital admission. Greater attention should be given to monitoring activity in this setting. Observation and self-report methods may encounter difficulties. Activity monitors (accelerometers) may offer a solution. This PhD thesis aimed to systematically review evidence investigating the validity of accelerometry to quantify purposeful activity within hospitalised adults experiencing acute or critical illness. It also aimed to investigate the validity of the Actigraph GT3X accelerometer in identification of body position (lying, sitting and standing) and step count in patients recovering from critical illness. Methods: A systematic review explored how accelerometer validity had previously been investigated within acute and critically ill hospitalised populations. Another study investigated the feasibility of the GT3X to identify body position and quantify typical activities undertaken by patients’ recovering from critical illness. Thirty healthy participants (mean age 58.8, SD 6.8) simulated this patient group, performing a movement protocol. Twenty ward based patients’ (mean age 62.3, SD 11.5), who had required prolonged ventilation in the ICU (≥ 48 hours) also completed a movement protocol containing typical daily activities. The validity of the GT3X to identify body position and step count was investigated using observation as the criterion measure. Results: A median (interquartile range) of Kappa = 0.94 (0.90, 0.98) for identification of body position was determined interpreting data from two GT3X accelerometers positioned in combination at the ankle and thigh. A mean difference (95% limits of agreement) of -0.84 steps (2.2 to -3.88) compared to observation was found for the ankle placement in step count quantification. Conclusions: The GT3X accelerometer is valid in identification of body position when positioned in combination on the thigh and ankle of the non-dominant leg in patients recovering from critical illness. An ankle placement is valid in quantification of step count.