Peace of Mind (De Tranquillitate Animi) is a dialogue written by Seneca the Younger during the years 49 to 62 A.D. It concerns the state of mind of Seneca's friend Annaeus Serenus, and how to cure Serenus of anxiety, worry and disgust with life.

For the modern reader, this short, powerful work offers insight into how to think like a Stoic. It is a road-map for guiding the mind to, in Seneca's words, "always pursue a steady, unruffled course... be pleased with itself, and look with pleasure upon its surroundings, and experience no interruption of this joy, but abide in a peaceful condition without being ever either elated or depressed."