Sicilian Defence, Najdorf Variation

The Najdorf Variation ( ) of the Sicilian Defence is one of the most respected and deeply studied of all chess openings. "Modern Chess Openings" calls it the "Cadillac" or "Rolls Royce" of chess openings. The opening is named after the Polish-Argentine grandmaster Miguel Najdorf, although he was not the first strong player to play the variation. Many players have lived by the Najdorf (notably Bobby Fischer and Garry Kasparov, although Kasparov would often transpose into a Scheveningen).
The Najdorf begins:
Black's 5...a6 aims to deny the b5-square to White's knights and light-square bishop while maintaining flexible development. If Black plays 5...e5 immediately, then after 6.Bb5+ Bd7 (or 6...Nbd7 7.Nf5) 7.Bxd7+ Nbxd7 8.Nf5, the knight on f5 is difficult to dislodge without concessions.
Black usually plans a to pressure White's e4-pawn. This is often carried out by means of ...b5, ...Bb7, and placing a knight on c5, or c4 via b6.
Variations.
Main line: 6. Bg5.
Classical Main line: 6...e6.
The main move. In the early days of the Najdorf 7.Qf3 was popular, but the reply 7...h6 did not allow White to obtain any advantage. Nowadays, White players almost universally respond with the move: 7. f4. White threatens 8.e5, but Black has several options:
Verbeterde List: 6...Nbd7.
Historically speaking, this was the usual reply until the mid-1960s, when the rejoinder 7.Bc4 put the move "out of business". Recently, however, ideas have been found by some Dutch players who call this variation "De Verbeterde List" ("The Improved Stratagem"). The idea for Black is to postpone ...e6 to retain more dynamic options (for example, to play ...e7–e5 in one move). The idea was tested by Petrosian, Belov, and others, but received popular attention and developed rapidly after use by Dutch player Lody Kuling in 2007.
The most important developments include:
English Attack: 6.Be3.
This has become the modern main line. Since the early 1990s, the English Attack, 6.Be3 followed by f3, g4, Qd2 and 0-0-0 in some order, has become extremely popular and has been intensively analysed. Four lines are then usual for Black:
If White plays 7.Nb3, then Black usually continues 7...Be6, trying to control the d5-square. The most common move is then 8.f3, allowing White to play Qd2 next move. If White had tried to play 8.Qd2, then Black could respond with 8...Ng4. 
But if White plays 7.Nf3, then Black's main choices are 7...Be7 and 7...Qc7.
Fischer–Sozin Attack: 6.Bc4.
Introduced by Veniamin Sozin in the 1930s, this received little attention until Fischer regularly adopted it, and it was a frequent guest at the top level through the 1970s. White plays 6. Bc4 with the idea of playing against f7, so Black counters with 6...e6 7.Bb3 b5. The Sozin has become less popular because of 7...Nbd7 where Black intends to follow up with ...Nc5 later. It is possible to avoid the 7...Nbd7 option with 7.0-0, but this cuts out the aggressive possibility of castling long.
Classical/Opocensky Variation: 6.Be2.
Because of the success of various players with these variations, White often plays 6. Be2 and goes for a quieter, more positional game, whereupon Black has the option of transposing into a Scheveningen Variation by playing 6...e6 or keeping the game in Najdorf lines by playing 6...e5. Another option is to play 6...Nbd7, in the spirit of The Verbeterde List; it is for this reason that this variation is called The Verbeterde List Unlimited.
Amsterdam Variation: 6.f4.
Some lines include:
GM Daniel King recommends 6...g6 against the Amsterdam Variation, leading to a more defensive kingside pawn structure. The idea is to eventually counterattack on the g1–a7 diagonal with a move like ...Qb6, preventing White from castling. An example line would be 6...g6 7.Nf3 Bg7 8.a4 Nc6 (note 8...Nc6 as opposed to the usual Najdorf ...Nbd7, as c6 is a more flexible square for the knight with a queen on b6) 9.Bd3 Qb6.
Adams Attack: 6.h3.
Introduced by Weaver Adams during the middle of the twentieth century, this odd-looking pawn move has mostly been used as a surprise weapon to combat the Najdorf. Should Black continue with 6...e5 anyway, White can respond with 7.Nde2 following up with g4 and Ng3, fighting for the weak light squares by playing g5. It is thus recommended that Black prevents g4 altogether with 7...h5.
Black can also employ a Scheveningen setup with 6...e6 followed by 7.g4 b5 8.Bg2 Bb7, forcing White to lose more time by defending the e4-pawn, since ...b4 is a threat. It was not until early 2008 that an answer to Black was finally found. After 9.0-0 b4, White has the positional sacrifice 10.Nd5!, which gives Black long-term weaknesses and an open e-file for White to play on. Since then, it has been popular on all levels of play.
Other sixth moves for White.
Beside the main lines mentioned above White has other options: 6.f3, 6.g3, and 6.a4 are less common, but are also respected responses to the Najdorf. Moves such as 6.Bd3, 6.Qf3, 6.Rg1 (the Petronic Attack), 6.Nb3, 6.a3, 6.h4, 6.Qe2, and 6.Qd3 are rarely played, but are not so bad and may be used for surprise value. Other rare moves include 6.g4, 6.Nf3, 6.b3, and 6.Bd2.