Publisher Summary This chapter presents the theory of the correlation between response time (RT) and intelligence. It aims to point out the critical features of the chronometric evidence that are considered in formulating a theory of the RT-IQ correlation. It also presents the two basic chronometric variables—RT and reaction time standard deviation (RTSD). Because the distribution of individual RTs is nonsymmetrical, the mean RT (RTm) is necessarily always a larger value than median RT (RTmd). Despite the near-perfect correlation between RTm and RTSD, it is postulated that RTSD is theoretically the more basic variable than RTm. The effect of increasing task complexity is to increase the slower RTs much more than the faster RTs.. Increasing task complexity has a constant multiplicative effect on all of a subject's RTs. This effect is reflected in the RTm and RTSD for any given subject and also for the differences among subjects. These RT effects are evident in all ECT paradigms that differ in task complexity or information processing load. The chapter explains the neural oscillation as the basis of intra-individual variability. The theory posits that intra-individual variability in RT reflects the length of the period. A longer period results in a larger RTSD.