Childhood obesity has become an emotive and serious problem in Europe. In the UK, it has generated a great deal of interest by the media, health professionals, food companies, scientists, and more recently, the government. Upward trends have been recorded in most countries, levels are interestingly variable, and the origins of this variation have not been explained. The role of physical activity and sedentary habits in childhood obesity is not clear. This is partly due to measurement problems. Accelerometry is offering some clearer insight into patterns and levels of activity that may be influential in the determination of children's long term energy balance. Examples of this research are presented. (Contains 1 figure.)