The libraries of the national museums in Spain were established during the 19th and 20th centuries. Their holdings are the result of a long tradition of collecting, and together constitute an invaluable bibliographic legacy. With the objective of bringing to light and publicising this rich heritage, these libraries have developed an art libraries network, BIMUS, whose first achievement has been to create an online union catalogue. The project will continue to develop in stages, and its eventual aim is to help the Spanish museum libraries become a reference point for research into art history and the cultural heritage.