Stoicism is a Hellenistic school of philosophy, developed by the Greek philosopher Zeno of Citium around 300 B.C., which teaches the development of self-control and fortitude as a means of overcoming destructive emotions in order to develop clear judgment and inner calm and the ultimate goal of freedom from suffering (see the section on the doctrine of Stoicism for more details).
Stoicism is not just a set of beliefs or ethical claims, however, but rather a way of life, involving constant practice and training, and incorporating the practice of logic, Socratic dialogue and self-dialogue, contemplation of death, and a kind of meditation aimed at training one's attention to remain in the present moment.
Stoicism was originally based on the moral ideas of the Cynic school (Zeno of Citium was a student of the important Cynic Crates of Thebes), and toned down some of the harsher principles of Cynicism with some moderation and real-world practicality. During its initial phase, Stoicism was generally seen as a back-to-nature movement, critical of superstitions and taboos (based on the Stoic idea that the law of morality is the same as Nature).
Zeno's successor was Cleanthes of Assos (c. 330 - 230 B.C.), but his most influential follower was Cleanthes' student Chrysippus of Soli (c. 280 - 207 B.C.), who was largely responsible for the molding of what we now call Stoicism. He built up a unified account of the world, consisting of formal logic, materialistic physics and naturalistic ethics. The main focus of Stoicism was always Ethics, although their logical theories were to be of more interest for many later philosophers.
Stoicism became the foremost and most influential school of the Greco-Roman world, especially among the educated elite, and it produced a number of remarkable writers and personalities, such as Panaetius of Rhodes (185 - 109 B.C.), Posidonius (c.135 - 50 B.C.), Cato the Younger (94 - 46 B.C.), Seneca the Younger (4 B.C. - A.D. 65), Epictetus and Marcus Aurelius.