One of the most vexing challenges facing developmental researchers today is the statistical modeling of two or more behaviors as they unfold jointly over time. Although quantitative methodologists have studied these issues for more than half a century, no widely agreed-upon principled strategy exists to empirically analyze codevelopmental processes. Indeed, the plethora of available options makes selecting a specific analytic approach both confusing and overwhelming. In this article, we argue that a key step in adjudicating among alternative modeling strategies is to embrace the concept of within- and between-person components of change over time. First, we define the disaggregation of effects in grouped data, and then we extend these concepts to repeated measures. Then we review several available modeling strategies that capture these effects to varying degrees and raise three issues that can help to guide practice.