As a literary device, an allegory in its most general sense is an extended metaphor. Allegory has been used widely throughout history in all forms of art, largely because it can readily illustrate complex ideas and concepts in ways that are comprehensible or striking to its viewers, readers, or listeners.

Writers or speakers typically use allegories as literary devices or as rhetorical devices that convey hidden meanings through symbolic figures, actions, imagery, and/or events, which together create the moral, spiritual, or political meaning the author wishes to convey.

One of the best-known examples of allegory, Plato's Allegory of the Cave, forms a part of his larger work The Republic. In this allegory, Plato describes a group of people who have lived chained in a cave all of their lives, facing a blank wall (514a-b). The people watch shadows projected on the wall by things passing in front of a fire behind them and begin to ascribe forms to these shadows, using language to identify their world (514c-515a). According to the allegory, the shadows are as close as the prisoners get to viewing reality, until one of them finds his way into the outside world where he sees the actual objects that produced the shadows. He tries to tell the people in the cave of his discovery, but they do not believe him and vehemently resist his efforts to free them so they can see for themselves (516e-518a). This allegory is, on a basic level, about a philosopher who upon finding greater knowledge outside the cave of human understanding, seeks to share it as is his duty, and the foolishness of those who would ignore him because they think themselves educated enough.