Social relationships are better understood through flexible statistical methods that are capable of modeling complex, interdependent processes that evolve and emerge over time rather than reducing their inherent complexity. It explains modeling techniques that is especially well suited to the task is dynamical systems models (DSMs). DSMs are a family of time series techniques that aim to depict data through differential equations or difference equations. The chapter then focuses on the application of DSMs for modeling co-regulation in romantic relationships. It also explains the concept of co-regulation and describes how theoretical constructs in co-regulation can be conceptualized from a dynamical systems perspective. The chapter provides an example application of a DSM to modeling co-regulation in ambulatory heart rate in a married couple. It discusses the important issues to consider when applying DSMs to modeling co-regulation and other complex forms of interdependence in social relationships.