Il pastor fido (The Faithfull Shepherd in Richard Fanshawe's translation) is a pastoral tragicomedy set in Arcadia by Giovanni Battista Guarini. One of the most famous plays of the 17th century, it appeared in over a hundred editions following its first publication in Venice in 1590. The first known performances took place at Ferrara (1595 or 96) and at Mantua (1598).
It inspired numerous madrigal composers, including Giaches de Wert, Claudio Monteverdi, Sigismondo d'India, Alessandro Grandi, Tarquinio Merula, and Heinrich Schütz; Philippe de Monte named a volume after the play. The first operatic treatment was Handel's HWV 8 in 1712.
The Il pastor fido sonatas for Musette de cour or flute published as Vivaldi's opus 13 are in fact by Nicolas Chédeville.