This paper studies the theoretical reflections on the role of biography within historiography developed in Diodorus’ Bibliotheke (10,12,1-3 Vogel = 10,27,1-3 Cohen-Skalli). It is shown that almost all the characteristics and functions claimed there for biography are referred to historiography in the various proems of the Bibliotheke. It is further argued that the theory as a whole is an adaptation and modification of the theory of encomium in Isocrates’ Evagoras. The result of this adaptation by Diodorus is not rhetorical historiography, which subordinates history to rhetorical aims, but historiography that intends to take over the educational function of epideictic. The paper also discusses how Ephorus, Timaeus and especially Polybius use the theory of encomium in defining their own concept of historiography and briefly sketches Plutarch’s adaptation of it in his theoretical statements on biography. Keywords: encomium, biography, historiography, Diodorus, Polybius, Ephorus, Timaeus, Plutarch.