Despite its prevalence in low-income populations, there has been little attention paid to how maternal depression influences mother-child conversations about emotions and low-income preschool children's developing emotion understanding. The importance of a secure attachment as a positive influence on emotion understanding has also been infrequently studied in lower-income families. This longitudinal study examined attachment security and maternal depression when children were age 2 as predictors of mother-child references to emotion in conversations, and children's emotion understanding when children were three. Maternal depression at age 2, but not at age 3, showed a direct, negative relation to children's emotion understanding at age 3, independent of mother-child references to emotion and attachment security. More securely attached dyads made more references to emotion in conversation, which, in turn, promoted children's emotion understanding. It was concluded that secure attachment relationships support children's emotion understanding by promoting mother-child discussion of emotions, while emotion understanding in preschoolers is directly impaired by maternal depression.