This paper presents a novel approach to extract and to characterize steady state interaction by means of closed-loop structural analysis. A steady state interaction measure equivalent to the relative gain array is obtained. Due to the broad derivation basis and structural significance, the new measure allows for unified characterization of steady state interaction and systematic analysis of the impact of loop interaction on decentralized control systems. Important issues, such as the absolute and relative interaction, the size and the direction of interaction, positive and negative interaction, one way interaction problem, and the usefulness as well as the limitations of steady state interaction measures are systematically addressed. The impact of steady state interaction on closed-loop performance and stability is analyzed on the basis of individual loops within a classical paradigm. The analysis applies to nonrational processes. The use of steady state interaction measures as tools for variable pairing and controller design to counteract loop interaction for closed-loop performance and stability is demonstrated.