He narrowly missed winning the Classical World Chess Championship 2004: the match was drawn 7–7 and so Vladimir Kramnik retained the title.

[1] Peter Leko was born into an ethnic Hungarian family in the city of Subotica, Yugoslavia, but moved to Szeged when he was one year old.

In 1994 he became a Grandmaster at the age of 14 years, 4 months and 22 days, at the time the youngest to have done so, breaking the record previously held by Judit Polgár.

[13][14] In 1997, Leko won in Cienfuegos (5/9)[15] and Yopal (6½/9)[16] and took fourth place at the traditional Tilburg chess tournament (7/11) establishing him as a rising star, climbing to 16th in the world rankings in the January 1998 list.

[20][21] Leko's run of form culminated in achieving his first major tournament win (5/7) at Dortmund, half a point ahead of Vladimir Kramnik.

In January 2000, Leko faced the winner, FIDE World Champion Alexander Khalifman in a six-game match held in Budapest, which he won 4½-1½.

[24] Leko qualified comfortably from the B Group in second place to Evgeny Bareev and faced Alexei Shirov in a four-game match.

The match was won by Leko 2½-1½, ensuring he became the official challenger to Vladimir Kramnik for the Classical World Chess Championship 2004.

[25] His Candidates triumph was followed up by leading the Hungarian team on board one to team silver in the 35th Chess Olympiad with an unbeaten run, along with sharing first (7/12) at Linares in early 2003 with Kramnik, half a point ahead of Anand and Kasparov, notably ending a ten super-tournament winning streak by Kasparov.

[26] This was shortly followed up by second place at the 12th Amber Melody tournament (Blind 6/11, Rapid 7½/11) a point behind Anand,[27][28] and a solid result (5/9) in Budapest.

He competed as part of an Armenia vs Rest of the World match, scoring 4/6 and finished in fourth place in Dortmund (held in the same format as the Candidates Tournament in 2002 but with play-offs for final positions).

[30] After several delays due to the collapse of the Einstein Group, who were originally to have organised the match,[31][32][33][34] Leko contested the Classical World Chess Championship 2004 held from September 25 to October 18, 2004, in Brissago, Switzerland.

[37] Leko equalised the match score in Game 5 after switching his White opening from 1.e4 to 1.d4 playing a Queen's Gambit Declined line that Kramnik was familiar with.

Kramnik believed he could construct a fortress and save a draw but Leko managed to find a way to convert the win.

In game seven, Leko was surprised by an unusual move order in the opening and simplified the position leading to a draw soon after, closing out the first half of the match with the score 3½–3½.

For the first time Kramnik allowed Leko to play the aggressive Marshall Attack variation of the Ruy Lopez.

In the penultimate Game 13, in search of a win, Kramnik switched opening again with Black to the dynamic Benoni Defence and launched a kingside attack in the middlegame while Leko's pieces were on the queenside.

Leko's plan to exchange off White's pieces and form a fortress did not work and Kramnik was able to break through the Black position.

[43] A 4/9 at Dortmund[44] was followed by competing for the FIDE World Chess Championship title in San Luis, Argentina having qualified as most recent "Classical" challenger, finishing fifth with 6½/14.

In 2006, Leko scored solid results at Corus (tied 7th, 6½/14), Morelia-Linares (fifth, 7½/14), a 4½-3½ win in the Miskolc Rapid match against Anatoly Karpov,[45] Dortmund (tied third, 4/7) and in sharing first place with 5½/9 at the inaugural Tal Memorial, he completed a career "Grand Slam" of major tournament victories at Dortmund, Linares, Corus and Tal Memorial.

The next year, Leko struggled at Morelia-Linares tying for last place and lost the Miskolc Rapid match 4½-3½ against Vladimir Kramnik soon after.

He comfortably won his matches against Mikhail Gurevich (+3−0=1) and Evgeny Bareev (+2−0=3), to qualify for the eight-player championship tournament.

[47] Shortly after the World Chess Championship 2007 was held in Mexico City from 12–30 September as an 8-player double round robin.

In 2008, Leko tied for fifth in a strong field at Corus with 7/13 but tied for last place in Morelia-Linares with 5½/14 and lost a Miskolc Rapid match 3½-4½ against 17-year-old Magnus Carlsen,[48] but rebounded 4 months later with his third victory at Dortmund (after 1999 and 2002) with 4½/7,[49] sixth place (4½/9) in a tough field at the Tal Memorial and securing individual gold on board 1 at the 38th Chess Olympiad held in Dresden.

He struggled at the Tal Memorial of 2009 scoring 3/9, broke even at Corus 2010 with 6½/13, lost what would be the final Miskolc Rapid match 3½-4½ against Boris Gelfand and came bottom of the Dortmund field with 4/10.

In September 2012 he entered the FIDE Grand Prix 2012-13 as a nominee of AGON, who owned the rights to the World Championship cycle.

In April 2013 at the Zug event, he shared seventh place with Sergey Karjakin and Anish Giri scoring 5/11.

Outside of the Grand Prix cycle, Leko helped the SHSM team secure third place at the European Chess Club Cup held in Eilat, Israel in October 2012, with a score of 3/6.

[63] Leko first represented Hungary aged 15 years and two months at the 31st Chess Olympiad, going on to make seven more appearances at the Chess Olympiad, winning two team silvers and an individual gold, as follows:[64][65] Leko also took part in numerous Team Chess Championships, both at World and European level (notably debuting at Debrecen in 1992 at the age of 13 years 2 months), with results as follows: Peter Leko is known for playing in a solid and steady manner, avoiding risks.

On the way to winning the prestigious Corus chess tournament in 2005, Leko defeated Viswanathan Anand with the black pieces.