A cornerstone of children’s socio-cognitive development is understanding that others can have knowledge, thoughts, and perceptions that differ from one’s own. Preschool-aged children often have difficulty with this kind of social understanding, i.e., they lack an explicit theory of mind. The goal of this dissertation was to examine the role mental state language as a developmental mechanism of children’s early understanding of their own mental states (i.e., their introspective ability). Specifically, it was hypothesized that (1) parents’ ability to recognize and appropriately label their children’s mental states and (2) children’s linguistic ability to distinguish between their mental states shapes the development of children’s introspective ability. An initial prediction of the first hypothesis is that parents should recognized differences in the development of children’s self- and other-understanding in order to better help their children’s introspective development. In support of this prediction, parents (N = 400, Mage = 58 months, Range = 28-93 months) reported that children’s understanding of their own knowledge was greater than children’s understanding of others’ knowledge. A prediction of the second hypothesis is that children’s linguistic ability to distinguish between and appropriately label their own mental states should determine their ability to make fined grained judgments of mental states like certainty. In support of this prediction, children’s (N = 197, Mage = 56 months, Range = 36-82 months) ability to distinguish between their own knowledge and ignorance states was associated children’s ability to engage in uncertainty monitoring. Together, these findings provide support for the association between children’s linguistic environment and ability and their introspective development.