For many lay persons (and also many scientists) the causes of the obesity epidemic are obvious. People develop obesity because they don’t do enough physical exercise, and they eat too much because they source their intake primarily from fast food outlets. Many government interventions to curb obesity target these things by encouraging people to engage in certain levels of activity (typically 150 mins per week of moderate to vigorous activity) and by banning adverts for fast foods to children. These policies have had virtually no impact on the progress of the obesity epidemic, leading to questions about whether the premise that inactivity and fast foods are really to blame (or whether people just don’t follow government guidance very much). This talk will address these two questions separately. In the first half I will look at what evidence there is that physical activity energy expenditure has declined over the last 30 years using various data sources but principally data from the IAEA doubly-labeled water database. In the second half I will address the role of fast foods, looking at spatial distributions of fast food outlets in relation to obesity levels, correlational data between intake of fast foods and weight status and some intervention trials.