Theory of mind, the ability to attribute mental states to ourselves and others, is crucial for human social interaction and has been argued to fully develop around the age of 4. However, recent research suggests that children can perform rudimentary, preverbal ToM inferences at an earlier age, indicating a discrepancy between this early, implicit ToM and a later mastery of explicit ToM tasks. Already in the second year of life children show competence in grasping what an agent knows and does (...) not know in preverbal communication, whereas explicit performance on ToM tasks can only be observed around the age of 4. One possible explanation is that children require an additional representational resource in order to be able to associate their preverbal understanding with explicit terms that can be used to denote it in standard ToM tasks. The purpose of this thesis is to extend research on ToM in the third year of life by investigating one possible representational resource, namely mental state language. Mental state language is considered crucial for ToM development as it is the first explicit means by which children can verbalize mental states and it plays an important role for both precursors and subsequent mastery of explicit ToM tasks. However, how exactly mental state language production reflects children's awareness and comprehension of mental states remains understudied. Harris, Yang, and Cui have argued that children use mental state terms early on and possess basic representations of their own knowledge and ignorance. Since both mental state terms and basic representations appear to be present at the same time, one might assume that children already possess associations between them. However, experimental tasks that investigate mental state verbs like `know' and `think' are only able to find competence to differentiate between them around the age of 5. Thus, the extent to which the appropriate use of mental state terms in conversation reflects comprehension of the denoted mental states remains unclear. Given that mental state language develops around the second and third year of life, one may gain insight into the role it plays for a transition from implicit to explicit competence by investigating abilities which, according to our current understanding, develop around the same time and show both preverbal and verbal aspects. Perspective-taking and implicit false-belief understanding are such abilities. Children show early preverbal perspective-taking competence at the age of 24 months, but only develop verbal perspective-taking competence around the age of 3. Previous work has identified a relationship between mental state language and verbal perspective-taking at the age of 30 months, but no study has investigated this relationship in connection to preverbal perspective-taking. Also in the area of falsebelief understanding studies have mostly investigated the relationship between mental state language and explicit false-belief tasks, but no study has examined how mental state language may relate to an implicit false-belief task. Finally, mental state language has been found to correlate with a number of different competencies, among them inhibitory control and general language, but no study has systematically investigated which of these competencies serve as precursors that predict children's later mental state language production. Determining precursors is relevant for understanding crucial aspects of mental state language development and how it might relate to children's preverbal representations of mental states. The aims of the present thesis were thus to investigate the relationship between children's mental state language production and their comprehension of mental state terms and mental states, to determine the role that mental state language production may play in preverbal and verbal perspective-taking and implicit false-belief understanding and finally, to identify the basic linguistic and cognitive skills that contribute to the development of mental state language. These aims were pursued through 2 studies. In study 1, children were assessed at two time-points. Once they were tested at the age of 24 months with measures of general language, cognitive and motoric development and inhibition skills. An additional time they were tested at 27 months in order to assess children's mental state language production, their preverbal and verbal perspective-taking skills, meta-cognitive awareness of ignorance, implicit false-belief understanding and finally a task that measured their ability to infer a speaker's need for information from a statement about knowing or not knowing the location of an item and a speaker's desire for an object from statements about wanting or not wanting a particular object. The aim was to determine the concurrent relationship of children's mental state language production and their ability to comprehend and use these terms on experimental tasks, to identify the concurrent relationship between mental state language and preverbal and verbal perspective-taking and implicit false-belief understanding and finally to determine which developmental skills at the age of 24 months are significant predictors of children's later mental state language production. Study 1 showed that 27-month-olds were already able to produce mental state terms and showed a basic understanding of their own knowledge and ignorance, but failed to show comprehension of epistemic mental state terms in experimental tasks. Thus, despite their early competence in producing epistemic mental state terms, children's comprehension of these terms appeared to be limited. The results of the first study also identified that mental state language production was related to children's own verbal level-1 visual perspective-taking, in particular to usage of the verb `know/don't know' in several naturalistic contexts, independent of general language. There was also continuity from children's preverbal understanding of perspectives to their ability to report their own perspective in a verbal task. However, there was no relationship between preverbal understanding of perspectives and mental state language production according to a parental questionnaire, suggesting that early preverbal understanding of perspectives is not related to the mere production of mental state terms unless children already produce these terms in appropriate contexts. In relation to implicit false-belief understanding, children's performance suggested that they followed a simple strategy of looking at the last object location instead of demonstrating false-belief understanding. Finally, study 1 also identified sentence production, inhibition and fine motor skills at 24 months as significant predictors of children's later production of mental state language at 27 months. In study 2, children's ability to infer a speaker's need for information was investigated further by administering a pragmatic inference task from study 1 to 2-, 3- and 5-year-olds and determining at which age children showed competence. Study 2 showed that despite being able to draw the right pragmatic inferences for the mental state verb `want' at the age of 2, only at the age of 5 children were able to draw the right pragmatic inferences for the mental state verb `know/don't know'. The overall results of this thesis implied that the crucial aspect of mental state language is not mere production of the verbs, but children's experience with a variety of contexts in which mental state verbs can be used, corresponding with a socioconstructivist approach to cognitive development. The number of naturalistic contexts related to children's metarepresentations of their own ignorance, to their ability of inferring a speaker's desire and their ability to verbalize their own perspective, thus suggesting that the conversational input children receive is one of the main ways in which mental representations become explicit. Furthermore, the theoretical view that children's preverbal competence may be better explained by a production rule approach instead of early conceptual understanding is discussed. (shrink)