The concept of reverse engineering has an interesting theoretical application to indexing, conceiving an index as a means of representing the semantic structure of a book. Indexes only ever achieve partial representation, because of their selectivity, and depend on the notion of relevance. Relevance originates in an indexer’s sense of what a book is about. If this sense is lacking, then indexing is problematic. This perspective is applied to the challenge of indexing on the World Wide Web. Maximalist and minimalist approaches are contrasted, and a way of handling the typically multithematic character of web pages is presented.