This book constitutes the refereed proceedings of the 20th British National Conference on Databases, BNCOD 20, held in Coventry, UK, in July 2003. The 20 revised full papers presented together with abstracts of 2 invited talks were carefully reviewed and selected from numerous submissions. The papers are organized in topical sections on XML and semi-structured data; performance in searching and mining; transformation, integration, and extension; events and transactions; and personalization and the Web.