On October 1, 1987 Southern California experienced the first earthquake for more than sixteen years that had sufficient strength to cause casualties and significant structural damage. The more important results of the event, and of the major aftershock which followed three days later, are summarised with particular reference to the successes in the mitigation of seismic effect which have been achieved both by the application of more stringent code requirements, in the case of new buildings, and by strengthening procedures in the case of existing ones.