The Sicilian Defence is a chess opening that begins with the following moves:

1...c5 is one of the most popular responses to White's first move 1.e4. Like 1...e5, the move contests control of the d4-square, but breaks symmetry immediately, often leading to dynamic and sharp positions.

The earliest recorded notes on the Sicilian Defence date back to the late 16th century by the Italian chess players Giulio Polerio and Gioachino Greco. It was extremely popular in the second half of the 20th century and was extensively played and analysed by many grandmasters, such as Bobby Fischer, Garry Kasparov, and Mikhail Tal.

By advancing the c-pawn two squares, Black asserts control over the d4-square and begins the fight for the centre of the board. The move resembles 1...e5, the next most common response to 1.e4, in that respect. Unlike 1...e5, however, 1...c5 breaks the symmetry of the position, which strongly influences both players' future actions. White, having pushed a kingside pawn, tends to hold the initiative on that side of the board. However, 1...c5 does little for Black's development, unlike moves such as 1...e5, 1...g6, or 1...Nc6, which either develop a minor piece or prepare to do so. In many variations of the Sicilian, Black makes a number of further pawn moves in the opening (for example, ...d6, ...e6, ...a6, and ...b5). Consequently, White often obtains a substantial lead in development and dangerous attacking chances.

Meanwhile, advancing a queenside pawn has given Black a spatial advantage there and provides a basis for future operations on that flank. Often, Black's c5-pawn is traded for White's d4-pawn in the early stages of the game, granting Black a central pawn majority. The pawn trade also opens the c-file for Black, who can place a rook or queen on that file to support the queenside counterplay. In many variations, White castles queenside to exploit attacking chances on the kingside at the cost of moving his king to the flank where Black has a spatial advantage.^{[citation needed]}

The Sicilian Defence was analysed by Giulio Polerio in his 1594 manuscript on chess, though he did not use the term 'Sicilian Defence'. It was later the subject of analyses by leading players of the day Alessandro Salvio (1604), Don Pietro Carrera (c. 1617), and Gioachino Greco (1623), and later Conte Carlo Francesco Cozio (c. 1740). The great French player and theoretician André Danican Philidor opined of the Sicilian in 1777, "This way of opening the game ... is absolutely defensive, and very far from being the best ... but it is a very good one to try the strength of an adversary with whose skill you are unacquainted."

In 1813, the English master Jacob Henry Sarratt effectively standardised his English translation of the name of this opening as 'the Sicilian Defence', referring to an old Italian manuscript that used the phrase il gioco siciliano ('the Sicilian game'). The Sicilian was fairly popular for much of the nineteenth century; Louis-Charles Mahé de La Bourdonnais, Adolf Anderssen, Howard Staunton, Louis Paulsen, and Carl Jaenisch all played it with some consistency. In the ninth edition of Modern Chess Openings, Walter Korn noted that the Sicilian "received three of its earliest practical tests, and a big boost in popularity, in the 1834 La Bourdonnais–McDonnell chess matches, 1843 Staunton–St. Amant match, and the London 1851 chess tournament." Staunton wrote of the Sicilian, "In the opinion of Jaenisch and the German Handbuch, with which I coincide, this is the best possible reply to 1.P-K4, [1.e4 in algebraic notation] 'as it renders the formation of a centre impracticable for White and prevents every attack'."

The opening fell out of favour in the later part of the nineteenth century, when some of the world's leading players rejected it. Paul Morphy, the world's best player in the late 1850s, decried "that pernicious fondness for the Sicilian Defense ... extending from about 1843 to some time after 1851". Wilhelm Steinitz, the first World Champion, also disliked the Sicilian and rejected it in favour of 1...e5. The death of the opening's two greatest proponents, Staunton and Anderssen, in 1874 and 1879 respectively, also contributed to its decline. It has been said that "these losses almost dealt a knockout blow to the Sicilian because it took a long time to find such important figures to carry the Sicilian's standard." George H. D. Gossip, in The Chess Player's Manual, first published in 1874, wrote, "Of late years ... discoveries have been made which have the effect of considerably strengthening White's attack, and the 'Sicilian' is now considered by most modern authorities to be a comparatively weak mode of play." Freeborough and Ranken, in their treatise Chess Openings: Ancient and Modern (1889, 1896), wrote that the Sicilian "had at one time the reputation of being the best reply to 1.P-K4 [1. e4], but this has not been confirmed by popular practice. Several eminent players have, however, held to the opinion that it is quite trustworthy."