Tooth decay is the commonest disease of childhood. We have known for over 90 years that fluoride can help reduce the risk of tooth decay. Water fluoridation has a 70-year history. We have a unique opportunity to study the impact of water fluoridation in West Cumbria, as a new plant will come on line in May 2013. This project aims to provide strong evidence of the effects and costs of a reintroduced water fluoridation scheme on young children. Water fluoridation works in two ways: Systemic (from birth) built into the teeth strengthening them as they develop. Topical strengthening the surface of the teeth after they have erupted in the mouth. The study design aims to assess the topical effects of water fluoridation by recruiting groups of children and following them over 6 years. We will recruit all children in their first school year in 2013 and examine their teeth at aged 5, 7 and 11-years-old. This will enable us to assess the topical effects alone of fluoridation on the permanent (adult) teeth. Our main outcome measure will be to compare the number of children who develop any tooth decay in fluoridated and non-fluoridated area.