The purpose of this paper is to examine the value of W. B. Yeats’s penultimate play, Purgatory. It is a one-act verse play. There are only two characters: An Old Man and A Boy. Its story is simple. The Old Man tells his son (A Boy) of his family’s past: the mésalliance of his mother and his father’s squandering of everything she had, which he considers pollution to his family and declares as a capital offence. The Old Man kills his son with the knife he used to kill his father to stop the pollution from passing on to the next generation, and to stop the ‘Dreaming Back’ process for his mother. Purgatory is as much a play about the end of a historical cycle as it is a personal story. The obvious decline of the old man’s family fortune is an image of a ruined Ireland, its vigour spent and its thought forced in upon its own past. The qualities that have caused Purgatory to be one of Yeats’s most admired plays is the condensation and compression of his material, coupled with a lucid and immediately accessible realistic plot. The characters, actions and images are both natural and symbolic, moving and meaningful. The real strength of Purgatory lies in its unobtrusive poetic quality, the harmony of realistic subject matter and symbolist design within a lyrical composition of undoubted concentration and power. In Purgatory, more than anything else, Yeats solved the problem of speech in verse drama, which is one of his contributions to modern drama. Instead of contrasting voice patterns, he unified the action with a freely varied verse form in iambic tetrameters which is admirably suited to the terse, sharp idiom of modern speech. The most remarkable feature of this very natural verse form is its ability to reflect emotional intensification as the rising dramatic action moves through contrast and reversal to its inevitable climax.