This chapter focuses on the study of interindividual differences in pattern development. This is logical in the present context in which generalizations aretobe made to populations of individuals andin which interindividual differences are thought to be profound and of prime interest in themselves. However, it should not be interpreted that we consider the study of the singleindividual tobe unimportant; on the contrary, it can be argued that this study is at the heart of understanding individual development, bothin its own right, and as a basis for careful generalization, in which studies of many such single cases are integrated. In the context of studying short-term changes in behavior, this argumentwas put forward by Jones and Nesselmade ( 1990). It is also relevant in the context of the study of long-term development (Nesselroade & Ghisletta, 2000; Valsiner, 1986). In this latter case, methodological tools such as latent growth curve analysis and time-series analysis are available for studying individual trajectories, as indicated in chapter 3.