The focus of this volume is methodological issues in research on individual developmental processes. A necessary prerequisite for the choice of appropriate methods for treatment of data in empirical research on individual development is a strong link between the methodology applied for elucidation of the problern focused on, on one hand, and the character of the structures and processes being investigated, on the other hand. The appropriate link between phenomena under investigation and the methods applied for treatment of data implies that the statistical model must match the psychological model for the phenomena under consideration. The appropriateness and effectiveness of the statistical model applied for the study of a specific problern depends on how weil the properties ofthe psychological model are reflected in the statistical model.