A force-based seismic assessment procedure for existing reinforced concrete moment resisting frames is discussed. The assessment procedure is based on determining the probable strength and ductility of the critical mechanism of post-elastic deformation of the frame. Account is taken of the likely seismic behaviour of reinforced concrete beams, columns and beam-column joints with substandard reinforcement details typical of structures designed before the 1970s, as determined by the results of experimental testing and analytical studies. The assessment aims at determining the available lateral load strength and structural (displacement) ductility factor of the frames so that the designer can determine the likely seismic performance of the structure by referring to acceleration response spectra for design earthquake forces for various levels of structural ductility factor.