AbstractMothers’ mental state language in conversation with their preschool children, and children's preschool attachment security were examined for their effects on children's mental state language and expressions of emotional understanding in their conversation. Children discussed an emotionally salient event with their mothers and then relayed the event to a stranger. Compared to mothers of insecurely attached children, mothers of securely attached children used more mental state language and had children who used more mental state language with both mother and stranger, and who expressed more emotional understanding in the mother–child conversation. Maternal mental state language and attachment security made shared contributions to children's mental state language with their mothers. Maternal mental state language accounted for the effects of attachment security on children's expressions of emotional understanding in the mother–child conversation. Mothers’ mental state language to their children may enhance secure attachment and foster children's understanding of mental states in self and others.