This chapter reviews the literature on processing speed, documenting the relationship between senescence and slowness. By nature, processing speed is networked. Most often, its measurement relies on sensory input, which is then “processed” by being reacted to in some way, and then that processing has to be demonstrated somehow, whether by writing or speaking or reacting. For example, when completing coding tasks, which are copying tasks used in general intelligence testing to measure processing speed, examinees look to a key at the top of the page to see the symbol associated with a certain number. Each number has its own symbol. They then reference that key when copying missing symbols in the space below under their corresponding number. This requires visual input perceived by the eye’s photoreceptors to travel down the length of the optic nerve, cross at the optic chiasm along its way toward the occipital lobe located in the back of the brain where it will be consciously processed. Moreover, performance is then reliant on white matter tracts traversing the parietal and temporal lobes and “the left middle frontal gyrus” (Turken et al., 2008). And this says nothing yet about the motor capacity requisite to record a response. Thus, this chapter on processing showcases the importance of myelinated tracts in integrating disparate regions and the consequences of their age-related degradation. This chapter also reviews some of the history of processing speed testing as an elementary cognitive task with a relationship to general intelligence. With this history and background, it is hoped that readers will understand the twofold importance of processing: There is first the straightforward need to rapidly take in vast quantities of information, making corresponding judgments in real time. Beyond this, and what makes processing and reaction rate ever more important, is its correlation to general intelligence.