This chapter discusses the "person approach" to studying developmental processes by focusing on the distinction and complementarity between this holistic-interactionistic framework and what has become designated as the variable approach. Particular attention is given to measurement models for use in the person approach. The discussion on the central theme of this chapter leads to the proposition that the person approach rests on two interdependent pillars: (1) the holistic-interactionistic framework for developmental processes and the principles of differentiation and crystallization within that framework, working according to the principle of self-organization, and (2) the application of a research strategy that includes a measurement model appropriate for the study of integrated, holistic processes in terms of patterns of working components. (Contains 2 figures and 2 notes.)