The acoustics and vibration of coupled structures can be efficiently modeled by means of energy flow theory, usually known as statistical energy analysis (SEA). SEA is based upon relations derived from the interaction of pairs of coupled modes and is applied empirically to coupled structures possessing multiple interacting modes. This talk investigates the accuracy of SEA models by deriving exact energy flow relations based upon modal models. Using white noise disturbances but otherwise deterministic structural models, we present energy flow relations for multiple coupled structures. These relations are formulated in terms of thermodynamic energy, which is a measure of subsystem energy dissipation relative to subsystem disturbance intensity. The resulting energy flow models are compared to SEA models based upon coupled, uncoupled, and blocked subsystem energy. The results are applied to systems of coupled oscillators as well as coupled beams to demonstrate predicted energy flow under both strong and weak coupling.