Childhood obesity is a global public health concern. In England, the prevalence of overweight/obesity increases from one fifth at the start of the primary school years to one third by the age of 10-11 years. This thesis examines the role of primary schools in preventing obesity. Stakeholder views are considered through a systematic review, and two qualitative studies investigating the perceptions of headteachers, parents and children. Data from a childhood obesity prevention trial (the WAVES study) are also used to examine the relationships between school policy/practice and pupil weight status/physical activity levels. Findings show that stakeholders support the school role in preventing obesity, and in helping families to lead healthier lifestyles, though limited expertise and resources are barriers. Although most schools actively promote health, there is much variation. For example, time allocated for physical education and breaks varies by school and has a significant impact on children’s physical activity levels, particularly for boys. In conclusion, school policies and practices can impact on children’s health, and schools are ideally placed to support families to prevent obesity. However, schools require support to perceive this role as a feasible and integral part of their function, rather than as an increasing burden of responsibility.