Johannes Gutenberg's printing press, developed around 1440, is often called the most important invention of the second millennium. Before printing, books were laboriously copied by hand, making them rare and expensive.
Gutenberg's innovation wasn't printing itself—the Chinese had block printing for centuries. His breakthrough was movable metal type: individual letters that could be arranged, printed, and reused. Combined with an adapted wine press and oil-based inks, this system could produce books at unprecedented speed.
The Gutenberg Bible, printed around 1455, demonstrated the technology's potential. Within decades, print shops spread across Europe. By 1500, an estimated 20 million volumes had been printed.