The cortex of the frontal lobes is 'motor' cortex in the broadest sense of the word. It is the peak of a hierarchy of anterior neural structures dedicated to the execution of actions. For the temporal organization of movements, the frontal cortex has at its disposal two cognitive functions that complement each other: memory and motor set, i.e. the preparation for movement. Their operation is especially apparent at the highest stage of the frontal motor hierarchy, which is the prefrontal cortex. Recent microelectrode studies in behaving monkeys reveal that during retention of sensory information for subsequent action, memory and set are supported by two distinct but intermixed populations of prefrontal neurons.