Pier Francesco Tosi (ca. 1653  – 1732) was a castrato singer, composer, and writer on music. Born in Cesena to Giuseppe Felice Tosi, a composer and organist, Tosi became one of the most famous castrati of his day. Tosi began his career singing in church choirs as a boy and young man. He sang in a Rome church, 1676–7, belonged to Milan Cathedral choir from 1681 until his dismissal for misconduct in 1685, made his one recorded appearance in opera at Reggio nell’Emilia in 1687, in Giovanni Varischino’s Odoacre, and was based in Genoa before going in 1693 to London, where he gave weekly public concerts and taught. From 1701 to 1723 he travelled extensively as musical and diplomatic agent of Emperor Joseph I and the Elector Palatine. From 1724 he again taught in London for some years; sometime before 1681 he had become a priest. Although he composed a number of cantatas and arias, he is best known as the author of Opinioni de' cantori antichi e moderni (1723), a treatise on singing. This was translated into English as Observations on the Florid Song by Johann Ernst Galliard in 1742 and into German as Anleitung zur Singkunst by Johann Friedrich Agricola (who also provided an extensive commentary of his own) in 1757. Tosi also wrote some vocal works. He died in Faenza.