This article begins by exploring ideas about physical exercise as outlined in the advice literature that circulated widely in late medieval and early sixteenth-century England. Whereas other aspects of these popular guides to health have attracted considerable interest on the part of medical and cultural historians, recommendations about exercise have been largely neglected. Yet it was deemed essential for both physical and mental wellbeing. Its principal function was to augment and redistribute the body's innate heat, which functioned as ‘Nature's primary instrument’, while improving digestion and the elimination of waste, encouraging restorative sleep, and combatting stress. These ideas spread rapidly throughout society, being harnessed to suit the practical needs of many classes of people, especially after the Black Death when keeping fit became a priority. Not even animals could escape current assumptions about the importance of remaining active, while political theorists emphasised the need to purge idle and unproductive elements from the communal body.