1. A method of generating a concept dictionary ( 140 ) for use in querying an information system ( 125 ), comprising the steps of: (i) receiving an information search criterion; (ii) deriving ( 105 ), using a lexical reference source ( 115 ), at least one search criterion having related meaning to said received search criterion ( 110 ); (iii) identifying sets of information in said information system ( 125 ) relevant to said received search criterion ( 110 ) and to said at least one derived search criterion; (iv) analysing the identified sets of information to derive relationships between said received search criterion ( 110 ) and said at least one derived search criterion in the context of said information system ( 125 ); and (v) storing, in a concept dictionary ( 140 ), information relating to said received ( 110 ) and said at least one derived search criterion and to respective said derived relationships therebetween, for use in querying said information system ( 125 ).