The Codex Leicester is a collection of writings by Leonardo da Vinci. It is named after Thomas Coke , later the 1st Earl of Leicester, who purchased it in 1717.

This remarkable 72-page manuscript is almost 500 years old and was handwritten in Italian by da Vinci, using his characteristic mirror writing, and supported by copious sketches. It covers a wide range of his observations and theories on astronomy, the properties of water, rocks and fossils, air and celestial light.

The Codex Leicester provides a rare insight into the inquiring mind of the definitive Renaissance artist, scientist and thinker as well as an exceptional illustration of the link between art and science and the creativity of the scientific process.

The Codex was purchased in 1980 by wealthy industrialist, art collector and philantropist Armand Hammer from the Leicester estate, and renamed the "Codex Hammer".

The Codex Hammer was purchased by Bill Gates in November 1994 for $30.8 million from Christie's. As of 2003 it was on display at the Seattle Art Museum.