This thesis investigates the significance of the notion of fantasia for artists and critics of art in Italy during the sixteenth century up to the definitive edition of Giorgio Vasari’s Lives (1568). It charts the pre-history of this term in an age before the imagination was attributed paramount importance in aesthetics, yet in a time when artists already intensively debated the mental powers at play in their practice. In this context, fantasia was a controversial faculty and an ambivalent term of ekphrasis. Its appeal as a philosophical concept was counteracted by its vexed role in the psychology of perception, where it was sometimes appraised as an uncontrollable pictorial agent that accidentally agglomerated mental images. In the wake of early Mannerist anti-classicism and its valuation of abnormality, fantasia was reclaimed as an irrational creative process allowing for formal renewal. The imagination, however, was not yet understood as ‘creative’ in the modern sense. Its potential for innovation was limited to the recombination of elements previously observed in nature. As the supremacy of mimesis (imitation) was increasingly put into question, the combinatorial model of invention fantasia stood for became a major paradigm through which the question of non-naturalistic form was problematized. The beliefs attached to fantasia, therefore, structured the early modern thinking about the nature of pictorial fiction, the value of artistic innovation and its relation to selfhood and subjectivity in the years leading up to the Council of Trent and the academisation of art. We argue that fantasia is essential to understanding some of the earliest aesthetic reflections on the unconscious dimension of creation and the share of unpredictability and accident inherent in any attempt at artistic renewal.