The fields of psychology and human development are experiencing a resurgence of scientific inquiries about phenomena that unfold at the level of the individual. This article addresses the issues of analyzing intraindividual psychological/developmental phenomena using standard analytical techniques for interindividual variation. When phenomena are characterized by intraindividual variation, analyses must be done at the level of the individual. Research on these processes generally uses methods analyzing interindividual variation, rather than intraindividual, however, a noteworthy fundamental difference exists between the two. The current article provides theoretical and empirical illustrations of the fundamental contrast between variation within subjects and variation between subjects. First, we explain this distinction through the classical ergodic theorems. Next, we discuss the utility of analysis of intraindividual variation for developmental processes. We then provide an empirical example, using recently-developed methods for analyzing person-specific developmental processes. This article presents for the first time a methodological approach in which first the factor model is determined by exploratory and confirmatory P-technique, and then in a separate second phase the dynamic model for the latent factor series is determined.