The general heading of seismic risk covers a variety of problems, of which the following at least may be distinguished: risk analyses for a particular structure on a particular site; 
 the determination of large-scale zoning schemes; 
 microzoning; 
 risk analyses for earthquake insurance ; 
 risk analyses for civil defence and hazard reduction programmes. 
 This paper is concerned chiefly with the first of these, although a few remarks concerning the others appear in the final section. Its intention is to identify the major uncertainties which enter into the estimation of seismic risk (in the sense of 1. above) and to place an order of magnitude estimate on the errors each is likely to cause. Some earlier papers with a similar theme are by Cornell and Vanmarke (1969), Donovan and Bernstein (1978), McGuire and Shedlock (1981), McGuire and Barnhard (1981) among others. Evidently there is a considerable element of personal judgement in such an attempt; the principal aim is to draw attention to the effects of aspects which are difficult to bring into explicit consideration and as a consequence are in danger of being ignored or otherwise brushed under the carpet.