1. A method of assigning a document to a cluster of documents containing related content, each cluster being associated with a cluster summary describing the content of the documents in the cluster, the method comprising: determining, at a document clustering system, whether the document should be grouped with one or more previously created cluster summaries, the previously created cluster summaries being stored in a memory in a B-tree data structure; and if it is determined that the document should not be grouped with the one or more previously created cluster summaries, then creating, at a document clustering system, a cluster summary based on the content of the document and storing the created cluster summary in the B-tree data structure.