The results of several thousands of inelastic time history analyses, which have been made on single degree of freedom structures to assess P-delta effects induced in earthquakes, are reviewed. The principal factors influencing P-delta actions are shown to be the ductility, the duration of the severe ground motion, the level of damping and the period of the structure. A method of designing for P-delta effects for single degree of freedom structures is presented. A limited number of analyses of multi-storey frames and walls indicate that the approach may be used for multi-storey structures. This paper gives background information on the P-delta method of analysis given in an appendix to the commentary of the proposed loading code.