This study examined the association between the security of attachment and processes influencing the development of emotion regulation in young children. A sample of 73 4 1/2-year-olds and their mothers were observed in an emotion regulation probe involving mild frustration for children, and mothers and children were later independently interviewed about how the child had felt. Fewer than half the mothers agreed with children’s self-reports in the emotion they attributed to children (a lower rate than the concordance of observer ratings with children’s self-reports), and higher mother-child concordance was associated with secure attachment and mother’s beliefs about the importance of attending to and accepting their own emotions. Mother-child conversations about recent events evoking children’s negative emotion were also analyzed. Children were less likely to avoid conversing about negative feelings when they were in secure attachments and when mothers were more validating of the child’s perspective. Children’s greater understanding of negative emotions was also significantly associated with higher mother-child concordance and less child conversational avoidance. Taken together, these findings underscore the multiple influences of attachment on emotion regulation and the importance of children’s emotion understanding to these processes.