Stoicism is a school of Hellenistic philosophy founded by Zeno of Citium in Athens in the early 3rd century BC. It is a philosophy of personal virtue ethics informed by its system of logic and its views on the natural world, asserting that the practice of virtue is both necessary and sufficient to achieve eudaimonia (happiness, lit. 'good spiritedness'): one flourishes by living an ethical life. The Stoics identified the path to eudaimonia with a life spent practicing virtue and living in accordance with nature.

Alongside Aristotelian ethics, the Stoic tradition forms one of the major founding approaches to virtue ethics. The Stoics are especially known for teaching that "virtue is the only good" for human beings, and that external things, such as health, wealth, and pleasure, are not good or bad in themselves (adiaphora) but have value as "material for virtue to act upon". Many Stoics—such as Seneca and Epictetus—emphasized that because "virtue is sufficient for happiness", a sage would be emotionally resilient to misfortune. The Stoics also held that certain destructive emotions resulted from errors of judgment, and they believed people should aim to maintain a will (called prohairesis) that is "in accordance with nature". Because of this, the Stoics thought the best indication of an individual's philosophy was not what a person said but how a person behaved. To live a good life, one had to understand the rules of the natural order since they believed everything was rooted in nature.

Stoicism flourished throughout the Roman and Greek world until the 3rd century AD, and among its adherents was Emperor Marcus Aurelius. It experienced a decline after Christianity became the state religion in the 4th century AD. Since then, it has seen revivals, notably in the Renaissance (Neostoicism) and in the contemporary era (modern Stoicism).