This study examined the relationship between the development of theory of mind and the development of the belief terms think, know, and guess. Children with autism and Asperger syndrome, matched to children with specific language impairment and normal development, completed false belief, belief term comprehension, and belief term expression tasks. The autistic group's performance on the false belief, belief term comprehension, and belief term expression tasks was significantly poorer than that of the Asperger, language impaired, and normal groups. Across groups an association was found between false belief and belief term performance. Results support a growing body of literature demonstrating links between the development of theory of mind and communicative competence.