© Springer Japan KK 2017. All rights are reserved. The primate prefrontal cortex (PFC) is composed of several different areas, which have different anatomical connections with other brain structures. We have studied the functional divisions among the prefrontal areas by combining lesion-behavioral experiments with single-cell recordings in intact monkeys using an analog of the Wisconsin Card Sorting Test as a behavioral paradigm. Our results suggest that the PFC is composed of multiple functional units, each of which plays different elementary roles in the performance of cognitively demanding tasks. These elementary functions are mutually dependent on one another, and then the overall performance of the PFC goes beyond a mere sum of the elementary functions. We have also obtained results suggesting that the control of cognitively demanding tasks depends on the posterior parts of the PFC when they are well learned. The most anterior part of PFC, the frontopolar cortex, starts to play another role, i.e., exploration of other possibilities than those pursued by the current task. By having this function of the frontopolar cortex, primates may have increased the flexibility and adaptability of their behaviors in changing environments.