Using a longitudinal, UK representative sample from the Millennium Cohort Study, the present study examined longitudinal variations in parent ratings of child social, emotional and behavioural difficulties and prosocial behaviour (preschool to end of Key Stage 1); the magnitude of parent-teacher agreement regarding behaviour ratings; and concurrent relationships between behaviour and language and literacy at the end of the Key Stage 1. The findings showed significant downward trends in ratings of young children's social and behavioural difficulties and an increase in ratings of prosocial behaviour over the 3-7 years period. Ratings of emotional difficulties remained fairly stable over the same period. Also, medium- to- strong language and literacy effects were found for behavioural difficulties (e.g. hyperactivity) and prosocial behaviour. Finally, teacher-parent agreement regarding behaviour ratings was found to be modest and higher for social and behavioural difficulties, hyperactivity in particular, than for emotional symptoms and prosocial behaviour. These findings have important implications for preschool and primary school policy and practice regarding children's social behaviour. (Contains 5 tables.)