MARTIN SLIWINSKI AND JACQUELINE MOGLE descriptive model of intraindividual change is by no means a trivial task. The purpose of this chapter is to critically examine different analytic approaches that researchers have used to describe intraindividual change in longitudinal aging studies. The first is a time-based approach that formally represents intraindividual change in some variable (eg, negative affect, memory) as a function of the passage of time. The second is a process-based approach that formally represents intraindividual change in some outcome as a function of concurrent (or lagged) changes in another variable. We examine some of the assumptions, the flexibility and the limitations of both of these approaches to modeling intraindividual change. This chapter is not intended to address the complexities of fitting specific statistical models to real data. Instead, this chapter addresses epistemological issues that pertain to the type of inferences supported by different approaches to testing hypotheses using longitudinal data. Issues pertaining to centering variables, estimation techniques, and model specification are all critically important for correctly interpreting results from fitting statistical models to longitudinal data. Any of several excellent