An important aspect of perspective-taking ability is the appreciation that mental states such as beliefs, desires, and knowledge change over time. The current study focused specifically on 3-, 4-, and 5-year-olds' understanding that they will have knowledge in the future that they do not currently possess--for example, that when they are grown-ups, they will know what the words on a newspaper say. We also compared this understanding to children's understanding that adults have knowledge that children do not. To address the possibility that children's correct responses stemmed from a general rule that "adults know everything," we also included questions to which adults could not know the answer. Results show that children's understanding that they will have knowledge in the future that they do not currently possess and that adults possess knowledge that children do not improves substantially during the preschool years. Moreover, only the 5-year-olds in our study acknowledged that certain things are "unknowable," even by adults. Finally, children's performance did not differ as a function of whether they were asked about their own future knowledge or an adult's current knowledge. Findings are discussed in terms of their implications for theories of perspective-taking and mental state reasoning, future thinking, and children's motivation to learn.