The aim of this article is to describe the intraindividual variability of intelligence. Psychological theories of intelligence grew up as an attempt to describe and account for the interindividual variability of cognitive skills. Therefore, the phenomenon of intraindividual variability of this trait was studied rather infrequently. In this chapter, two types of such changes are distinguished. First, there are long term changes, like developmental, regressive, and degenerative. Second, there are short term changes, called 'temporary disconnections' and constant fluctuations of intellectual skills. The chapter ends with some conclusions for psychological theory and practice, which result from the necessity to take into account the phenomenon of the intraindividual variability of human intelligence.