In this paper we present a new technique for detecting changes on the Web. We propose a new method to measure the similarity of two documents, that can be efficiently used to discover changes in selected portions of the original document. The proposed technique has been implemented in the CDWeb system providing a change monitoring service on theWeb. CDWeb differs from other previously proposed systems since it allows the detection of changes on portions of documents and specific changes expressed by means of complex conditions, i.e. users might want to know if the value of a given stock has increased by more than 10%. Several tests on stock exchange and auction web pages proved the effectiveness of the proposed approach.