The distinctive, philosophically interesting concept of eternity arose very early in the history of philosophy as the concept of a mode of existence that was not only beginningless and endless but also essentially different from time. It was introduced into early Greek philosophy as the mode of existence required for fundamental reality (being) contrasted with ordinary appearance (becoming). But the concept was given its classic formulation by Boethius, who thought of eternity as God’s mode of existence and defined God’s eternality as ‘the complete possession all at once of illimitable life’. As defined by Boethius the concept was important in medieval philosophy. The elements of the Boethian definition are life, illimitability (and hence duration), and absence of succession (or timelessness). Defined in this way, eternality is proper to an entity identifiable as a mind or a person (and in just that sense living) but existing beginninglessly, endlessly and timelessly. Such a concept raises obvious difficulties. Some philosophers think the difficulties can be resolved, but others think that in the light of such difficulties the concept must be modified or simply rejected as incoherent. The most obvious difficulty has to do with the combination of atemporality and duration. Special objections have arisen in connection with ascribing eternality to God. Some people have thought that an eternal being could not do anything at all, especially not in the temporal world. But the notion of an atemporal person’s acting is not incoherent. Such acts as knowing necessary truths or willing that a world exist for a certain length of time are acts that themselves take no time and require no temporal location. An eternal God could engage in acts of cognition and of volition and could even do things that might seem to require a temporal location, such as answering a prayer. The concept of God’s eternality is relevant to several issues in philosophy of religion, including the apparent irreconcilability of divine omniscience with divine immutability and with human freedom.