This paper examines the relationship between parents’ and children’s language skill for a nationally representative birth cohort born in the UK – the Millennium Cohort Study (MCS). We investigate both socio-economic and ethnic differentials in children’s vocabulary scores, and the role of differences in parents’ vocabulary scores in accounting for these. We find large vocabulary gaps between highly educated and less educated parents, and between ethnic groups. Nevertheless, socio-economic and ethnic gaps in vocabulary scores are far wider among the parents than among their children. Parental vocabulary is a powerful mediator of inequalities in offspring’s vocabulary scores at age 14, and also a powerful driver of change in language skills between the ages of five and 14. Once we account for parental vocabulary, no ethnic minority group of young people has a negative ‘vocabulary gap’ compared to whites.