If you are talking about # of books / materials it would be United States Library of Congress. Physical size ... not sure which is largest.

According to Guinness, The Library of Congress, Washington D.C.

The United States Library of Congress in Washington D.C. ranks as the largest library in the world. The Guinness Book of World Records currently lists the Library of Congress as the "World's Largest Library". This apparently is based on the shelf space the collection occupies; the Library of Congress states that its collection fills about 530 miles (850 km), while the British Library, reports about 388 miles (625 km) of shelves. The Library of Congress holds about 130 million items with 29 million books against approximately 150 million items with 25 million books for the British Library.

The two largest I have seen are the Bibliotheque nationale de France and the Vatican Library. The most beautiful hand written and drawn books are the incunabula at the Vatican. Best regards.

The Library of Congress is the largest library in the world by shelf space and holds the largest number of books according to Wikipedia. The Top 10 largest are as follows: Top 10 List on the Ten Largest Libraries in the World http://www.clipmarks.com/clipmark/6A03F215-C089-4070-A536-AEB0966A2A5C/ 10. New York Public Library New York City, NY, USA Founded in 1895, this library stocks over 11 million books. 9. Vernadsky National Scientific Library of Ukraine Kiev, Ukraine Founded in 1919, this library stocks over 13 million books. 8. Harvard University Library Cambridge, MA, USA Founded in 1638, this library stocks over 13.1 million books. 7. Institute for Scientific Information on Social Sciences of the Russian Academy of Sciences Moscow, Russia Founded in 1969, this library stocks over 13.5 million books. 6. British Library London, England Founded in 1753, this library stocks over 16 million books. 5. Deutsche Biblothek Frankfurt, Germany Founded in 1990, this library stocks over 18.5 million books. 4. National Library of Canada Ottawa, Canada Founded in 1953, this library stocks over 18.8 million books. 3. Library of the Russian Academy of Sciences St. Petersburg, Russia Founded in 1714, this library stocks over 20 million books. 2. National Library of China Beijing, China Founded in 1909, this library stocks over 22 million books. 1. Library of Congress Washington DC, USA Founded in 1800, this library stocks over 29 million books. What's with the all caps and no question mark?

Going by the definition of a library. Library: a collection of information, sources, resources, and services: it is organized for use and maintained by a public body, an institution , or a private individual. The answer would be the "INTERNET" In the more traditional sense, a library being a collection of books . Then it would be "The Library of Congress"

Alexandria