This article is a plea to cognitive developmental psychology to show a genuine and durable interest in individual variability, be it inter- or intraindividual. Individual differences have indeed often been relegated to domains of application whereas intraindividual variability has been considered as measurement noise. Moreover, interindividual differences are most frequently considered only as quantitative variations around a same norm (e.g., are individuals faster or slower in their development/aging?). In contrast, it is here argued that individual variability constitutes an important fundamental phenomenon, which would help us better understand processes underlying cognition as well as cognitive development. Also, one should look for qualitative differences, that is, for differences in the way individuals develop or age. Illustrations are provided of erroneous or only partial conclusions that were reached because of an over reliance on cross-sectional, univariate experimental designs.