Processing speed (PS) is an individual cognitive ability that measures the speed with which individuals execute cognitive tasks, particularly elementary cognitive tasks. PS has been proposed to be a key cognitive component, along with working memory, and is psychologically and clinically important. Various types of speed training affect performance of untrained cognitive measures. In this article, we review studies of PS training or training involving speeded tasks and describe the methodologies along with the psychological and neuroimaging findings related to PS training. There are various types of PS (speed) training tasks. Evidence indicates that PS training can enhance performance on untrained speeded tasks. However, the extent of transfer may vary depending on the methodology. A particular type of speed training seems to affect mental health in older adults. Neuroimaging studies of speed training have shown that the effects of speed training on neural mechanisms may vary depending on the training tasks. Adaptive procedures to modulate the difficulties of training tasks based on a subject's performance by modulating the task speed can be applied to various cognitive tasks, and these procedures can perhaps be used to develop training protocols for enhancing various cognitive functions.