This chapter discusses measuring of intelligence by Francis Galton, J. McK. Cattell, and Alfred Binet. Charles Spearman abhorred the program that would separate the mind into a loose confederation of independent faculties of learning, memory and attention. Although most intelligence researchers today probably accept that the general factor is to stay, they remain sharply divided on its explanation. These disagreements go well beyond a rejection of Spearman's specific suggestions that g is either mental energy or the eduction of relations and correlates. Spearman saw that he needed to provide a psychological or (better still) a neurobiological explanation of g. The two favorite paradigms for this program of research were inspection time (IT) and choice reaction time (RT). Aided by the new technologies of brain imaging, research on intelligence, working memory, and other so-called executive functions has begun to point to some of the brain structures common to them all.