Definitions for the required and available ductility used in seismic design are discussed. Methods for estimating the yield deformation and the maximum available deformation are described and suggestions are made for appropriate definitions. Examples are given of different imposed histories of inelastic displacement which have been used in the experimental testing of structures and structural assemblages in which cycles of quasi-static loading are applied. A quasi-static procedure for establishing the available ductility factor of a subassemblage by laboratory testing is recommended.