Everyday behaviors require a high degree of flexibility, in which prior experience is applied to inform behavior in new situations. Such flexibility is thought to be supported by memory integration, a process whereby related experiences become interconnected in the brain through recruitment of overlapping neuronal populations. In this talk, I will discuss our work demonstrating that memory integration relies on hippocampal–prefrontal circuitry and allows for acquisition of new knowledge beyond what we directly experience. I will show how such knowledge is flexibly deployed to promote new learning and higher-level cognitive functions such as reasoning and concept formation. I will also discuss developmental data exploring the relationship between maturation of hippocampal-prefrontal circuitry and the emergence of reasoning ability. I will show that children and adolescents are less likely to link new experiences to their existing memories, limiting their ability to reason about the relationships among distinct events.