Everyday life requires frequent shifts between cognitive tasks. Research\nreviewed in this article probes the control processes that reconfigure\nmental resources for a change of task by requiring subjects to switch\nfrequently among a small set of simple tasks. Subjects' responses\nare substantially slower and, usually, more error-prone immediately\nafter a task switch. This 'switch cost' is reduced, but not eliminated,\nby an opportunity for preparation. It seems to result from both transient\nand long-term carry-over of 'task-set' activation and inhibition\nas well as time consumed by task-set reconfiguration processes. Neuroimaging\nstudies of task switching have revealed extra activation in numerous\nbrain regions when subjects prepare to change tasks and when they\nperform a changed task, but we cannot yet separate 'controlling'\nfrom 'controlled' regions.