Different terms have described quick responding, processing speed, cognitive speed, psychometric speed, and perceptual speed. Methodology derived from speeded tasks assumes that cognitive processes intervening between stimulus and response can at least be relatively isolated by appropriate manipulation of experimental conditions. The most comprehensive body of data assembled to test the theory that processes responsible for speed on elementary cognitive tasks (ECTs) are the same as those responsible for complex intelligent actions from Jensen's studies of simple and choice reaction times (RTs). Although the extent to which inspection time (IT) and RT measure the same or different processes is still an open question, there is compelling evidence that correlation between IQ and processing speed estimated by IT or choice RT reflects shared genetic influences. There is considerable evidence that white matter lesions are associated with slower processing speed and poorer performance on tests of attention and memory.