Il pastor fido is a pastoral tragicomedy by the Italian poet Battista Guarini (1538–1612). It was written in the 1580s while Guarini served as court poet for Duke Alfonso d'Este II in Ferrara, and published in 1589.

      The play itself is very charming, and the style of Guarini's writing exceptionally elegant. Before the play was even published, however, it sparked a fierce critical debate between the Ancients and the Moderns that lasted for decades (even centuries). In its first round (1586–93), Giasone Denores, a professor of moral philosophy in Padua, represented the Ancients, while Guarini himself stood for the Moderns. The debate focused on several key issues: Guarini's mixing of the fundamentally distinct classical genres of tragedy and comedy into a new hybrid genre, "tragicomedy;" the overly sophisticated rhetoric used by the pastoral shepherds and nymphs; its immorality; and its lyrical style of writing, which was seen as ill-suited for the stage.

      Yet, like so many controversial literary works, Il pastor fido turned out to be immensely popular in sixteenth-century Italy, and it quickly became one of the most widely read texts across Europe through the following century. It also became the inspiration for many great musical works, including the madrigals of Luca Marenzio, Giaches de Wert, Claudio Monteverdi, Heinrich Schütz, and many others.