For the determination of the dynamic response of a structural
system to earthquake ground motion the mode superposition technique offers an alternative approach to the well established direct integration method. Whereas the principle of modal superposition is encountered commonly in elastic analyses, the response of a yielding structure is predicted almost universally by direct integration of the
equation of motion. In this paper the principles of modal superposition are extended into the post-elastic domain. Among the advantages which accrue from the modal based transformation are a potential reduction in the number of dynamic degrees-of-freedom considered in the solution, and a new insight into the structure response provided by the instantaneous dynamic properties of the structure.