Mental health problems often debut in early childhood and may last throughout adulthood, thereby making early detection and intervention especially important. The overarching aim of the present thesis was to identify patterns of emotional and behavioural problems indicating mental health problems in preschool children. To facilitate the detection of such problems early on, one available screening instrument Strengths and Difficulties Questionnaire (SDQ), was validated. The development and interaction of externalising problems in preschool children were studied over time. Functioning and behaviour and their relations to protective and risk indicators in both environmental and personal characteristics were explored. The long-term goal was to increase knowledge about early identification of emotional and behavioural problems in preschool children in order to facilitate early intervention. In Study I (n=690), the subscales Hyperactivity and Conduct Problems were shown to be valid for children in the age group 1–3 years. A reasonable level of validity was found for the age group 4–5 years when using the original SDQ four-factor solution. The preschool teachers considered most of the SDQ items relevant and possible to rate. Based on the results of Study II (n=815), a score of ?12 on the SDQ Total Problems Scale is recommended as a cut-off for Swedish preschool children. There were significant differences between boys and girls on all subscales except for the Emotional subscale. The Swedish norms for SDQ are to a large extent similar to findings from other European countries. Study III (n=195) showed that preschool children’s conduct problems decrease over time. Children exhibiting more initial hyperactivity (at year 1) have less reduction in conduct problems over time, i.e. the more hyperactivity early in life, the more conduct problems at year 3. In Study IV (n=197), children high in engagement and social interaction function well over time, even in the presence of hyperactivity, while children with low engagement and interaction alone or in combination with hyperactivity and conduct problems continue to have problems. Stability was related to the existence of a larger number of protective or risk indicators respectively. Taken together, this thesis has shown that the SDQ can be used to identify preschool children at risk of developing mental health problems later in life.