|Abstract||Maternal input is thought to play an important role in young children's development of mental state language (i.e. words referring to desires and beliefs). Maternal mental state language is thought to influence children's own mental state language and socio-cognitive understanding (e.g. theory of mind), but the mechanism is unclear. In addition, the association between security of attachment and mental state language has been examined but has failed to yield consistent results. The current study examines the longitudinal association between mother and child mental state language in the context of the attachment relationship. Further, the study examines whether maternal mental state language functions in the zone of proximal development, in which mothers scaffold children's mental state language according to Vygotskian criteria. Results reveal partial support for a Vygotskian framework such that mother's desire talk decreases while belief talk increases over time. In addition, a significant three-way interaction for attachment security x person x type suggests important differences in mental state discourse for secure and insecure attachment dyads. Implications for a Vygotskian framework are discussed.