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