This longitudinal study combined, in a single study, different aspects of children's knowledge about mental phenomena and thus could investigate relations among the development of language, theory of mind, and later metamemory. In total, 183 German children were tested at ages 3, 4, and 5. Each time of testing included a set of theory-of-mind tasks, a battery of language development, and additionally, at Time 3, a set of metamemory questions. The findings demonstrate strong relationships between children's language abilities and their theory of mind (both first- and second-order false beliefs). Moreover, both theory-of-mind and language competencies significantly predicted later metamemory, with their relative contribution changing over time. Language may influence metamemory developmentally both directly and indirectly (through theory of mind).