Despite a surge of scholarly and public interest in the mid-twentieth century, Mannerism remains an ill-defined and problematic period label. The first goal of my thesis is to define the style in its chronology and stylistic attributes. Noting its artificiality and the influence of self-fashioning, I identify its clearest definition in Giorgio Vasari's writing and art. Second, I discuss the use of the sophisticated style by the artist and his patron, Cardinal Alessandro Farnese, as a means of social advancement and legitimization. Finally, I analyze the iconography and style of the Sala dei Cento Giorni in the Palazzo della Cancelleria, Rome as the collaborative apex of the self-fashioning of Vasari and Farnese.