Guillén de Castro's play La fuerza de la costumbre (1625) depicts the process of re-teaching gender to Hipólita and Félix, a sister and brother who have grown up performing the gender opposite to their physical sex. This dissertation provides the first edition of the play since 1927, and the first ever critical edition, which contains notes, critical apparatus and essays, and compiles information from all extant sources, including the manuscripts (not used in the 1927 edition). The 1625 print edition serves as the base text, with variants from the four manuscript witnesses compiled in an index. The critical apparatus includes a biography of the author, textual history, editorial methodology, metrical analysis, bibliography and notes on the text. / Romance Languages and Literatures