The purpose of this paper is to examine the theme of The Only Jealousy of Emer, one of W. B. Yeats's 'Cuchulain plays'. The central action of the play is the struggle of three women—Emeer, Eithne Inguba and Fand—for possession of Cuchulain. Unlike Eithne Inguba's confused, cowardly action, Emer's behavior is brave as well as insightful. And as the chorus suggests, Fand's allurements are transitory. Fand's metallic allurement contrasts with Emer's passionate suffering. Fand wants to catch him to fulfill herself, not to aid in his salvation. Emer is more courageous than Eithne Inguba, more self-sacrificing than Fand, and more forgiving than Aoife. Emer's love for her husband transfigures her, whereas Aoife's vindictive hatred for Cuchulain costs them their only child. Emer is certainly a Yeatsian heroine who performs as nobly as Deirdre or Cuchulain. Yeats's most immediate source for his Cuchulain plays was Lady Gregory's Cuchulain of Muirthemne, but he significantly altered the source to serve his purposes. Emer's thwarted desire to attack Fand with her knife is one of the few links between Yeats's source and his much changed finished work of art. From this primitive tale of vengeance and jealousy, Yeats created a sophisticated drama of mental suffering and self-sacrifice. A second major change in the source involves Cuchulain's recollection of Fand's attempt to ensnare his soul. Both his fear upon awakening and his later praise of Emer for saving him suggest that he is glad of his deliverance, not despondent over the loss of Fand. Yeats's greatest modification came in his treatment of Emer's temperament. Instead of the jealous wife of seeking vengeance for herself, she is jealous only for her husband's well-being. By renouncing the love of the man she needs to end her loneliness, Emer proves herself superior to the source heroine. In the final version, Yeats dramatized, through Emer's hope for the return of Cuchulain's love for her, through her initial inability to give up her hope of winning back his love, and through her final renunciation of his love, the depth of her love and the extent of her sacrifice.