Close relationships are described in terms of the temporal coordination of behavior based on the similarity of partners’ internal states. Coupled nonlinear dynamical systems were used to model the emergence, maintenance, and disruption of coordination in such relationships. For each system, there was a control parameter corresponding to an internal state and a dynamical variable corresponding to behavior. Computer simulations investigated how the temporal coordination of behavior in a relationship reflects the similarity of partners’ control parameters and the strength of (...) coupling. Several types of coordination were observed, with in-phase synchronization occurring for strong coupling and similarity in internal states. In a variation of the model, each system could adjust its own control parameter to synchronize its dynamics with that of the other system. Simulation results provide insight into several topics in the study of close relations and group dynamics. (shrink)