1. Apparatus for identifying topics of document data, the apparatus comprising: a word ranker operable to rank words that are present in or representative of the content of the document data; a co-occurrence ranker operable to rank co-occurrences of words that are present in or representative of the content of the document data; a phrase ranker operable to rank phrases in the document data; a words selector operable to select the highest ranking words; a co-occurrence identifier operable to identify which of the highest ranking co-occurrences contain at least one of the highest ranking words; a phrase identifier operable to identify the phrases containing at least one word from the identified co-occurrences; a phrase selector operable to select the highest ranking one or ones of the identified phrases as the topic or topics of the document data; and an outputter operable to output data relating to the selected topics.