The series of ten paintings known as 'The Labours of Hercules' (1634, Museo Nacional del Prado), commissioned and executed for the decoration of the Hall of Realms in the Buen Retiro Palace, is one of Francisco de Zurbarán’s most famous artistic projects. This article offers new hypotheses regarding a key aspect of the cycle’s production: the corpus of visual sources that Zurbarán used both to represent the actions and physical appearance of Hercules and other characters. Among its contributions, this article discusses a series of prints by Hendrick Goltzius, whose direct relation to the Prado cycle had not been previously considered.