A prerequisite step for studying the magnitude and meaning of IQ change is to distinguish between true IQ change that is a researchable phenomenon and IQ "change" that can be accounted for by measurement error. We studied the reliability, magnitude and meaning of IQ change using scores on the WISC--R obtained from a representative sample of 794 children at ages 7, 9, 11 and 13. The findings suggest that, in the majority of children, IQ change is either negligible in amount, unreliably measured or both. In a nontrivial minority of children, naturalistic IQ change is marked and real, but this change is variable in its timing, idiosyncratic in its source and transient in its course. We discuss the implications of these findings for interventions that aspire to improve IQ scores.