Library of Congress Subject Headings (LCSH) are increasingly seen as 'the' English language controlled vocabulary, despite their lack of a theoretical foundation, and their evident US bias. In mapping exercises between national subject heading lists, and in exercises in digital resource organization and management, LCSH are often chosen because of the lack of any other widely accepted English language standard for subject cataloguing. It is therefore important that the basic nature of LCSH, their advantages, and their limitations, are well understood both by LIS practitioners and those in the wider information community. Information professionals who attended library school before 1995 - and many more recent library school graduates - are unlikely to have had a formal introduction to LCSH. Paraprofessionals who undertake cataloguing are similarly unlikely to have enjoyed an induction to the broad principles of LCSH. This is the first compact guide to LCSH written from a UK viewpoint.