Stephen Robertson is a British computer scientist. He is known for his work on probabilistic information retrieval together with Karen Spärck Jones and the Okapi BM25 weighting model. Okapi BM25 is very successful in experimental search evaluations and found its way in many information retrieval systems and products, including open source search systems like Lucene, Lemur, Xapian, and Terrier. BM25 is used as one of the most important signals in large web search engines, certainly in Microsoft Bing, and probably in other web search engines too. BM25 is also used in various other Microsoft products such as Microsoft SharePoint and SQL Server.