Physical activity (PA) monitoring in children remains an important challenge for epidemiologists, exercise scientists, clinicians and behavioural researchers as they attempt to utilize PA in combating child-related disorders such as obesity in children and young adults. Monitoring PA in children using wearable sensors provides an objective means of assessment which could significantly improve the accuracy of assessment as compared to subjective methods of assessment. This chapter presents a case study utilizing inertial sensor technology to monitor the severity of idiopathic toe walking found in children. A major challenge in deploying portable sensors for monitoring PA in children is the interpretation of sensor data, in particular correlating the data to a physiological measurement which describes biological changes in the body or even to changes in behavioural patterns. Global positioning system-based monitors have been used to measure the distance covered by a subject over a given period of time and hence, it is assumed, children total energy expenditure.