In this thesis I investigate the concept of sophrosyne (prudence, temperance, self-control etc.) in those Platonic dialogues which are commonly considered to belong to the early or middle period of Plato's career as a writer. My hypothesis is the that there is a closer connection between the different accounts and descriptions of the virtue than is normally thought. My point of departure and the spine of the study is the Charmides, which is devoted as a whole to the question of how to define sophrosyne. In particular, I compare the different definitions proposed in this dialogue with the accounts and descriptions of sophrosyne in the Republic, arguing that most or all of the definitions from the Charmides can be taken to point to essential aspects or conditions of sophrosyne in the magnus opus. While the concept of sophrosyne cannot be separated from Socratic-Platonic philosophy as a whole, the related concepts of love (eros and filia), self and knowledge are especially close associates of our concept; the study therefore tries to pay these notions their due respect. A consequence is that references to the Lysis, the Symposium and the Phaedrus often figure alongside the aforementioned works. On the whole, the study attempts to bring into dialogue the Charmides and the Republic in particular, while drawing heavily on the other works mentioned.