In 1971, graduate student Penny Patterson began teaching sign language to a gorilla named Koko, unaware that this relationship would come to define both their lives. What started out as a scientific experiment evolved into an intimate friendship, which for almost half a century has challenged the way we think about animals and changed the course of many lives. Koko: The Gorilla Who Talks is a unique window into an incredible relationship. What can we learn from this extraordinary science experiment, and does it tell us more about animals’ emotions or our own? Over forty years later, now internationally famous, Koko continues to redraw the line between people and animals.