TESS is a longitudinal study following the mental health, psychosocial development and health behaviour of a sample of around 1,000 children born in Trondheim, Norway. In 2007, all Trondheim children born in 2003/2004 and their parents were invited to participate. After the parents completed a screening assessment for emotional and behavioural problems (Strengths and Difficulties Questionnaire version 4–16) for the children, the study selected a screen-stratified subsample of 1,250 children to participate, where children with emotional or behavioural problems were oversampled. The participants have been followed up every two years since the children were 4 years old.