"Person-oriented research" is based on the fact that statements at the aggregate level rarely apply at the level of smaller numbers of cases or individuals. The first tenet of person-oriented research proposes that structure and development of behavior are, at least in part, specific to the individual. While aggregate-level statements may be correct, they cover only that portion of the overall variability that is shared by the aggregate. Larger, and more important portions of variability may remain unexplained. Recent research has already created many examples that show that individual characteristics may not be captured at all when results that were created at the aggregate level are used for description at the individual level. With this volume, we pursue three aims: provide an overview of theoretical and methodological developments of person-oriented research; present recent and new developments in the domain of statistical analysis of data from a person-oriented perspective; and, present empirical sample cases in which methodological implications of the relationship between aggregate-level and person-oriented research are defined and illustrated.