Similar to adult obesity, childhood obesity has recently become a serious threat to public health. Children face the same environmental and lifestyle challenges that have made high energy dense food and drink increasingly available and opportunities for energy expenditure through transport and active play less likely. Governments are becoming concerned but research has been under funded, is relatively scarce in the UK and has provided little sound evidence on which to base interventions. Physical activity seems to be declining and there is some evidence that time spent in sedentary pursuits such as TV watching is increasing and that this may be related to obesity. However, not all children are inactive and it may be more fruitful to independently examine different elements of activity such as play, walking and cycling as transport, organised sports, and active jobs so that children's profiles of activity can be mapped on an individual and group level. Recent research using accelerometry is providing insight into those elements which produce significant energy expenditure in a child's daily routine. The school and the structure to the school day appear particularly important. It is also likely that attention will need to be paid to parenting--the anxieties that parents demonstrate when making decisions about the freedom of their child to be active. Furthermore, without neighbourhoods that are safe and attractive for activity it is unlikely that levels will be increased. For physical activity to contribute to reversing the trend in childhood obesity, a multi-factorial and multi-agency solution will be required that involves schools, the home, the neighbourhood and considered local planning and provision.