The human pupil is a privileged neurobiological window into emotion. The pupil is a neuro-emotional biomarker and an integral component of facial expression. Understanding its dynamics deepens our knowledge of the very nature of human communication, defends Prof. Freitas-Magalhães, the world's leading expert for Neuroscience and Biobehavioral Psychology and the Encyclopedia of Human Behavior, by Elsevier, Oxford, in his pioneering and extraordinary book on the science of the human pupil.