This thesis includes one qualitative literature review and five empirical studies examining aspects of a) the relationship between bone mass and physical activity in children aged 8-11 years using pedometry and accelerometry, and b) the variability and reliability of the RT3 accelerometer as a means of assessing physical activity patterns in children. Previous research has shown a positive relationship between physical activity and bone mineral density in children. However, study design appears to be confounded by the accuracy of measures of physical activity limiting conclusions that can be drawn. Prior to our investigation research investigating the relationship between bone mass and habitual physical activity measured by objective means in children is scarce, and no study had evaluated the technical performance of the R T3 accelerometer; a small lightweight triaxial accelerometer. To examine the relationship between bone mass, physical activity and calcium intake methodology included; the objective assessment of habitual physical activity initially by pedometry and finally by accelerometry; dietary analysis of calcium intake; and dual energy X-ray absorptiometry measured bone area, mineral content and density of the whole body and proximal femur. To examine the variability and reliability of the RT3 accelerometer methodology included; the assessment of intraand inter-monitor variability by vibration assessment at increasing Hz levels; intermonitor reliability and variability by inducing a variety of human motions in a laboratory based environment; and epoch selection was assessed comparing the 60 to one second time sampling modes using children in a field setting. The main findings were: a) steps per day, assessed by pedometry, explained a significant proportion of the variance in bone mass at the hip in children; b) the RT3 triaxial accelerometer was reliable across trials, although the anterioposterior vector recorded counts consistently higher than the mediolateral and vertical vectors; c) interunit variability of the RT3 was evident, particularly as activity intensity increased; d) use of the 60 second epoch setting may lead to inaccuracies when assessing activity of a vigorous intensity and above; and e) vigorous physical activity, assessed by the RT3 accelerometer, and calcium intake have an interactive effect on bone mass in children, whereby bone mineral content is only high when both vigorous activity and calcium intake are high. In conclusion, this thesis has highlighted quality control procedures that need to be in place when using the RT3 accelerometer and presented evidence for a synergistic action of vigorous physical activity and calcium intake on bone mass in children.