The Italian composer Claudio Monteverdi (1567–1643), in addition to a large output of church music and madrigals, wrote prolifically for the stage. His theatrical works were written between 1604 and 1643 and included operas, of which three—L'Orfeo (1607), Il ritorno d'Ulisse in patria (1640) and L'incoronazione di Poppea (1643)—have survived with their music and librettos intact. In the case of the other seven operas, the music has disappeared almost entirely, although some of the librettos exist. The loss of these works, written during a critical period of early opera history, has been much regretted by commentators and musicologists.

Opera, as a musical and theatrical genre, began to emerge during the early part of Monteverdi's career, initially as a form of courtly entertainment. With other composers, he played a leading part in its development into the main form of public musical theatre. His first opera, L'Orfeo, written in 1607 for the Mantuan court, which employed him, was a major success. In the years that followed, at Mantua and in his later capacity as maestro di cappella (director of music) at St Mark's Basilica in Venice, Monteverdi continued to write theatrical music in various genres, including operas, dances, and intermedi (short musical interludes inserted into straight plays). Because in Monteverdi's times stage music was rarely thought to have much utility after its initial performance, much of this music vanished shortly after its creation.

Most of the available information relating to the seven lost operas has been deduced from contemporary documents, including the many letters that Monteverdi wrote. These papers provide irrefutable evidence that four of these works—L'Arianna, Andromeda, Proserpina rapita and Le nozze d'Enea con Lavinia—were completed and performed in Monteverdi's lifetime, but of their music, only the famous lament from L'Arianna and a trio from Proserpina are known to have survived. The other three lost operas—Le nozze di Tetide, La finta pazza Licori and Armida abbandonata—were abandoned by Monteverdi before completion; how much of their music was actually written is unknown.

Background Monteverdi's creative life covered more than 50 years. Between 1590 and 1612 he served as a musician in the Gonzaga court in Mantua, followed by 30 years (1613–43) as maestro di capella at St Mark's Basilica in Venice. This timespan saw opera develop, from its beginnings as a limited form of court entertainment, to become part of the mainstream of public musical theatre. Before the Italian word "opera"—short for opera in musica ("musical work")—came into general use around 1634, musical stage works were typically termed favola in musica (musical fable), dramma in musica (musical drama), or tragedia in musica ("musical tragedy"); Monteverdi used these and similar descriptions for many of his early operatic projects.

The first work now generally considered as an opera is Jacopo Peri's Dafne of 1597, closely followed by Euridice (1600), for which Peri and Giulio Caccini wrote separate musical settings. Ottavio Rinuccini was the librettist for both Dafne and Euridice. In the new genre a complete story was told through characters, and in addition to choruses and ensembles, the vocal parts included recitative, aria and arioso. This was a development from various older forms of musical theatre that had existed since the earliest years of the Italian Renaissance; such forms included the maschera ("masque"), the ballo (a dance entertainment, often with sung passages), and particularly the intermedio or intermezzo, a short dramatic musical episode inserted as a prologue or entr'acte between the acts of straight plays. Another format in the later renaissance period was the torneo, or "tournament", a stylised dramatic spectacle in which the main singing was performed by a narrator. Sub-operatic forms of dramatic music continued to thrive as opera itself developed; the blurred boundaries that existed for many years between these forms and "opera" has led to debate about how to categorise some works. For example, the precise genre of Monteverdi's Il combattimento di Tancredi e Clorinda (1624) has proved particularly difficult to define.

Monteverdi's first acknowledged opera is L'Orfeo (1607). He composed, in all, 24 works for the stage. Of these, ten are usually classified as operas, of which the music for seven has been lost apart from a few fragments. Most of what is known about the missing works comes from surviving librettos and other documentation, including Monteverdi's own extensive correspondence. Tim Carter, a leading Monteverdi scholar, suggests that the high rate of loss is explicable because, in Monteverdi's times, "memories were short and large-scale musical works often had limited currency beyond their immediate circumstances".

For Mantua Monteverdi wrote six acknowledged operas for the Mantua court, of which only L'Orfeo survives with libretto and music intact. Four of the five lost Mantuan works were written after the composer had left the service of the Gonzagas in 1612 and was ensconced in Venice, but still retained contacts with Mantua. L'Arianna and Andromeda were completed and performed; the others were all abandoned incomplete.

L'Arianna (1607–08) (English: "Ariadne")

L'Arianna was composed as a festive piece for the wedding of the heir to the duchy, Francesco Gonzaga, to Margherita of Savoy, in May 1608. Monteverdi received the commission following L'Orfeos successful premiere at the court in February 1607. The libretto for L'Arianna was by Rinuccini, whose literary skills had earlier impressed Duke Vincenzo I of Mantua after a performance of Euridice. The composition of L'Arianna became a fraught affair for Monteverdi, being only one of three works that the duke required from him for the wedding—he had also to compose a musical prologue for Giovanni Battista Guarini's play L'idropica, and write the music for a dramatic dance, Il ballo delle ingrate. His life had been disrupted by the fatal illness of his wife Claudia; she died on 10 September 1607, but Monteverdi was given no respite by the duke. L'Arianna was largely composed in the last two months of 1607, an exertion that Monteverdi's biographer Hans Redlich describes as "superhuman". Monteverdi felt slighted by the lack of acknowledgement from the duke for his efforts; nearly 20 years later, in a letter to the Mantua court secretary Alessandro Striggio the Younger, he wrote that he had almost killed himself when writing L'Arianna in such a hurry.

Rinuccini used numerous classical sources as the basis for his libretto, in particular works of Ovid—the Heroides and the Metamorphoses—and poems from Catullus. After a prologue, the main action begins as Venus tells Cupid that Ariadne and her lover Theseus, fleeing from Crete after his slaying of the Minotaur, will shortly arrive in Naxos. Theseus, she reports, will then abandon Ariadne, as he believes her to be unacceptable to the people of Athens as their queen. Venus plans to match her instead with the god Bacchus, and asks Cupid to arrange this. Theseus and Arianna arrive; Theseus agonises over his decision to abandon her, but is advised by his counsellor that he is wise in his resolve, and departs. In the morning Ariadne, finding herself abandoned, searches vainly for Theseus on the shore, where she sings her lament. A fanfare indicates an imminent arrival; Ariadne hopes this is Theseus returning, but it is Bacchus and his entourage. Jupiter speaks from the heavens, and amid festive scenes Bacchus promises Ariadne immortality with the gods in return for her love.

Rinuccini extended the libretto during the rehearsals, after complaints from the duchess that the piece was "too dry"; as a result the early scene between Venus and Cupid, and Jupiter's blessing from heaven, were added. Preparations for the opera's performance were disrupted when, in March 1608, the leading soprano Caterina Martinelli died of smallpox. A replacement had to be found rapidly, and the title role fell to Virginia Andreidi, a renowned actress-singer who used the stage name "La Florinda"; she reportedly learned the part in only six days. In his analysis of Monteverdi's theatrical works, Carter suggests that the lament may have been added to the work to make the most of Andreini's acting and vocal abilities. The premiere, on 28 May 1608, was staged in a specially erected temporary theatre, which according to contemporary reports could hold an audience of several thousands. The production was lavish; apparently 300 men were required to manipulate the stage machinery. Federico Follino, who prepared the Mantuan court's official report on the occasion, praised the beauty of the work, the magnificence of costumes and machinery, and the sweetness of the music. Monteverdi's fellow-composer Marco da Gagliano was equally complimentary, writing that the opera had "visibly moved the entire audience to tears." It is possible that L'Arianna was performed in Florence in 1614; a projected performance in Mantua in May 1620 to celebrate Duchess Caterina's birthday was cancelled for unknown reasons. Otherwise, there are no records of the opera's performance before its revival in 1640 at the Teatro San Moisè, Venice. In his study of late Renaissance opera, Gary Tomlinson surmises that the work's enthusiastic reception in Venice was a significant factor in Monteverdi's decision to resume opera composition during his final years.

Of the music, only the lament survives. It was published independently from the opera in various forms; an adaptation for five voices was included in Monteverdi's Sixth Book of Madrigals in 1614, and two versions of the original solo were published in 1623. Other composers emulated the lament's format; Redlich asserts that it initiated a musical subgenre that lasted to the end of the 17th century and beyond. The libretto has been preserved; versions were published in Mantua in 1608, and in Venice in 1622 and 1639.

Le nozze di Tetide (1616–17) (English: "The Marriage of Thetis")

After Duke Vincenzo's death in February 1612, Monteverdi found himself out of favour at the Mantuan court. Vincenzo's successor Francesco had no high regard for Monteverdi, and dismissed him from his post. Upon Francesco's sudden death in December 1612, the dukedom passed to his brother Ferdinando, but Monteverdi was not recalled to the court and was appointed maestro di capella in August 1613 at St Mark's, Venice. However, he remained in contact with Striggio and other highly placed Gonzaga courtiers, through whom he was able to secure occasional commissions to compose theatrical works for the Gonzaga court. Thus, late in 1616, Striggio asked him to set to music Scipione Agnelli's libretto Le nozze di Tetide, as part of the celebrations for Duke Ferdinando's forthcoming marriage to Catherine de' Medici. This story, based on the wedding of the mythical Greek hero Peleus to the sea-goddess Thetis, had previously been offered to the Mantuan court by Peri, whose setting of a libretto by Francesco Cini had been rejected in 1608 in favour of L'Arianna.