Yawning makes one yawn, crying makes one cry. In the same way, a shiver, appetite, sexual desire and confidence are transmitted from one person to another. These examples capture the contagion-like dimension of emotion, spreading rapidly among people with tangible behavioural manifestations. Emotional contagion still challenges scientific explanation, and philosophical, scientific and anthropological topics converge around this issue. In Medieval Latin, there is a specific name for this contagion: compassio ('compassion'). Etymologically, 'compassion' means the co-experience of a 'passion', involving an involuntary reaction of the soul or the body imitating the reactions of others. The book investigates how these topics were treated in medieval learned texts, and illuminates the twofold enigma, that of the trajectory of the term compassio, and that of explaining the phenomenon it denoted.