Twelfth Night is Shakespeare's comedy of mistaken identity and misdirected love, written around 1601. Shipwrecked twins Viola and Sebastian each believe the other drowned. Viola disguises herself as a young man, Cesario, and enters Duke Orsino's service – where she falls in love with him, while carrying his love-messages to the Countess Olivia, who promptly falls for Cesario. Around this triangle runs the gulling of Olivia's puritanical steward Malvolio, tricked by a forged letter into believing his mistress loves him. Sebastian's arrival untangles everything into marriages – except for Malvolio, who exits swearing revenge.

Twelfth Night was written around 1601–1602, at the close of Shakespeare's great run of romantic comedies and alongside Hamlet. The earliest recorded performance is 2 February 1602 at the Middle Temple, noted in the diary of the law student John Manningham. Shakespeare drew the twins-and-disguise plot from Barnabe Riche's tale "Of Apolonius and Silla" and, behind it, the Italian comedy Gl'Ingannati ("The Deceived"). The play was first printed in the First Folio of 1623.

Twelfth Night is a romantic comedy – for many critics the finest of Shakespeare's festive comedies, the term C. L. Barber (1959) coined for plays built on holiday licence and the world turned upside down. But it is a comedy with a dark lining: the revelry runs alongside grief (both Viola and Olivia begin the play mourning brothers), the jokes on Malvolio tip into cruelty, and Feste's closing song sends the audience out into the rain. That bittersweet balance is why it is often called Shakespeare's farewell to comedy.

Twelfth Night is the festival on the eve of Epiphany (5–6 January), the final night of the Christmas season – traditionally a feast of misrule, when normal hierarchies were turned upside down and servants might play masters. The play never mentions the holiday, but it is steeped in its spirit: disguise, revelry, a steward dreaming of marrying his mistress. Its alternative title, What You Will, is the only subtitle Shakespeare ever gave a play – a shrug that suits a comedy where nearly everyone mistakes what, and whom, they want.

Twelfth Night is one of the most commonly taught Shakespeare comedies: a frequent choice at Key Stage 3 and GCSE in the UK, and a set text at A-Level, on AP English Literature (US), the IB Diploma, and undergraduate courses worldwide. This study guide is written to support all of these levels, with modern English translation, scene-by-scene analysis, character profiles, theme guides, and key quotes.

Twelfth Night opens on one of Shakespeare's most famous lines and never lets up. Among the most quoted: "If music be the food of love, play on", "some achieve greatness, and some have greatness thrust upon 'em", "Make me a willow cabin at your gate", "She sat like patience on a monument", "Love sought is good, but given unsought better.", and "the whirligig of time brings in his revenges". Each is analysed in depth on the key quotes page.