Before printing presses were invented, every book had to be copied out by hand, word by word, page by page. The people who did this were called scribes, and the hand-written books they produced are called manuscripts. Many of the library's manuscripts are illustrated with tiny, incredibly detailed pictures painted in brilliant colours, gold, and silver - a style of art called illumination.
The library has kept some of its oldest documents in special temperature-controlled rooms to protect them from heat, light, and moisture. Some manuscripts are so fragile that they can only be handled by experts wearing cotton gloves. Researchers from universities all around the world travel to Vatican City to study them.
In recent years, the library has started a huge project to photograph and digitise its collection - turning the pages of ancient books into digital images that anyone in the world can look at online. So far, tens of thousands of manuscripts have been scanned, and the project is still going. A manuscript that was hidden away in a vault for centuries can now be read by a child anywhere on Earth with an internet connection.