Numbers are of paramount importance in our scientifically and technically advanced culture. The successful processing of numerical information requires a suite of cognitive processes, such as categorization, working memory, and decision making, the core topics of the current FOR 5159. Numerical competence therefore offers a window of opportunity to study high-level cognitive functions that typically emerge from the prefrontal cortex and related association cortices. With the current project, we explore the neuronal underpinnings of working memory and decision processes required for cardinal (numerical quantity) and ordinal assignments (numerical rank) in trained macaque monkeys. We hypothesize that prefrontal networks become flexibly re-arranged to support changing cognitive demands during manipulation of numerical information ‘in the mind’. This approach will contribute to a better understanding of how the primate brain gives rise to behavioral flexibility.