In 1953 a young female Japanese monkey called Imo began washing sweet potatoes in water prior to eating, presumably to remove dirt and sand grains. Soon other monkeys had copied her behaviour and potato-washing gradually spread throughout the entire troop. When, three years after her first invention, Imo devised a second novel foraging behaviour, that of separating wheat from sand by throwing mixed handfuls into water and scooping out the floating grains, she was almost instantly heralded around the world as a monkey genius. Imo is probably the most celebrated of animal innovators. In fact, many animals will invent new behaviour patterns, adjust established behaviours to a novel context, or respond to stresses in an appropriate and novel manner. Innovation is an important component of behavioural plasticity, vital to the survival of individuals and potentially of critical importance to those endangered or threatened species forced to adjust to changed or impoverished environments. In this text, Simon Reader and Kevin Laland provide a comprehensive study of this subject.