This is a critical edition of this play by Calderon, based primarily on the text of the "primera parte" of 1635, but also taking into account three other 17th-century printed editions and a scribal manuscript in the Biblioteca Nacional of Madrid. The relationship between these texts is discussed in the introduction, which also deals with the source of the legend of Saint Patrick's purgatory. The introduction also considers the literary sources of the play, notably "Vida y Purgatorio de San Patricio", the date of composition, versification, and the ways in which the the play has been staged.