The Encyclopaedia of Chess Openings (ECO) is a reference work describing the state of opening theory in chess, originally published in five volumes from 1974 to 1979 by the Yugoslavian company Šahovski Informator (Chess Informant). It is currently undergoing its fifth edition. ECO may also refer to the opening classification system used by the encyclopedia.

Both ECO and Chess Informant are published by the Belgrade-based company Šahovski Informator. The moves are taken from thousands of master games and from published analysis in Informant and compiled by the editors, most of whom are grandmasters, who select the lines which they consider most relevant or critical. The chief editor since the first edition has been Aleksandar Matanović (1930-2023). The openings are provided in an ECO table that concisely presents the opening lines considered most critical by the editors. ECO covers the openings in more detail than rival single volume publications such as Modern Chess Openings and Nunn's Chess Openings, but in less detail than specialized opening books.

The books are intended for an international audience and contain only a small amount of text, which is in several languages. The bulk of the content consists of diagrams of positions and chess moves, annotated with symbols, many of them developed by Chess Informant. Chess Informant pioneered the use of Figurine Algebraic Notation to avoid the use of initials for the names of the pieces, which vary between languages.

Instead of the traditional names for the openings, ECO has developed a coding system that has also been adopted by other chess publications. There are five main categories, "A" to "E", corresponding to the five volumes of the earlier editions, each of which is further subdivided into 100 subcategories, for a total of 500 codes. The term "ECO" is often used as a shorthand for this coding system. ECO code is a registered trademark of Chess Informant.

• English Opening
• Benoni Defence
• Dutch Defence
• Réti Opening
• Benko Gambit
• Old Indian Defence
• Bird's Opening
• Anti-Indian systems (Trompowsky Attack, Torre Attack, Richter–Veresov Attack)
• Irregular openings, etc.

• Sicilian Defence
• Caro-Kann Defence
• Pirc Defence
• Alekhine's Defence
• Modern Defence
• Scandinavian Defence, etc.

• Ruy Lopez
• French Defence
• Petrov's Defence
• Vienna Game
• Centre Game
• King's Gambit
• Philidor Defence
• Italian (Giuoco Piano, Evans Gambit, Hungarian Defence, and Two Knights)
• Scotch Game
• Four Knights Game, etc.
• King's Pawn Opening, etc. (covers unusual/rare 1. e4 openings such as the Elephant Gambit and Ponziani Opening).

(including Grünfeld Defence but not other Indian Defenses)

• Queen's Gambit
  • Accepted
  • Declined (Slav, Orthodox, Tarrasch, Tartakower, Albin Countergambit, etc.)
• Grünfeld Defence
• Queen's Pawn Game, etc.

(other than Grünfeld Defence and Old Indian Defence)

• Nimzo-Indian Defence
• Queen's Indian Defence
• King's Indian Defence
• Catalan Opening
• Bogo-Indian Defence, etc.

• White first moves other than 1.e4, 1.d4 (A00–A39)
• 1.d4 without 1...d5, 1...Nf6 or 1...f5: Atypical replies to 1.d4 (A40–A44)
• 1.d4 Nf6 without 2.c4: Atypical replies to 1...Nf6 (A45–A49)
• 1.d4 Nf6 2.c4 without 2...e6, 2...g6: Atypical Indian systems (A50–A79)
• 1.d4 f5: Dutch Defence (A80–A99)

• 1.e4 without 1...c6, 1...c5, 1...e6, 1...e5 (B00–B09)
• 1.e4 c6: Caro–Kann Defence (B10–B19)
• 1.e4 c5: Sicilian Defence (B20–B99)

• 1.e4 e6: French Defence (C00–C19)
• 1.e4 e5: Double King Pawn games (C20–C99)

• 1.d4 d5: Double Queen Pawn games (D00–D69)
• 1.d4 Nf6 2.c4 g6 with 3...d5: Grünfeld Defence (D70–D99)

• 1.d4 Nf6 2.c4 e6: Indian systems with ...e6 (E00–E59)
• 1.d4 Nf6 2.c4 g6 without 3...d5: Indian systems with ...g6 (except Grünfeld) (E60–E99)

The first edition was published in the following years:

• Volume A: 1979
• Volume B: 1975
• Volume C: 1974
• Volume D: 1976
• Volume E: 1978

Volume D (1976) Code Author 00–01 Bagirov, Vladimir 94–95 Bagirov, Vladimir 60–65 Barcza, Gedeon 60–65 Florian, Tibor 70–79 Botvinnik, Mikhail 98–99 Botvinnik, Mikhail 70–79 Abramov, Lev 98–99 Abramov, Lev 56–57 Bukić, Enver 02–05 Cvetković, Srđan 02–05 Sokolov, Vladimir 31–32 Euwe, Max 50–55 Filip, Miroslav 45–46 Gheorghiu, Florin 45–46 Samarian, Sergiu 38–39 Gipslis, Aivars 80 Gipslis, Aivars 90–91 Gipslis, Aivars 58–59 Hort, Vlastimil 66–69 Judovič, Mihail 85–89 Karpov, Anatoly 43–44 Korchnoi, Viktor 47–49 Korchnoi, Viktor 15–19 Kotov, Alexander 33–34 Larsen, Bent 30 Matanović, Aleksandar 35–36 Matanović, Aleksandar 30 Ugrinović, Dragan 35–36 Ugrinović, Dragan 25 Matulović, Milan 25 Krnić, Zdenko 06–09 Minev, Nikolay 37–38 Minev, Nikolay 40–42 Parma, Bruno 26–29 Polugaevsky, Lev 96–97 Smyslov, Vasily 10–14 Suetin, Alexey 20–24 Taimanov, Mark 81–84 Uhlmann, Wolfgang 92–93 Uhlmann, Wolfgang

Volume E (1978) Code Author 00–05 Averbakh, Yuri 00–05 Neishtadt, Yakov 32–37 Bagirov, Vladimir 67–69 Boleslavsky, Isaac 67–69 Lepeshkin, Vladimir 27–29 Botvinnik, Mikhail 47–49 Botvinnik, Mikhail 27–29 Abramov, Lev 47–49 Abramov, Lev 97–99 Byrne, Robert 97–99 Mednis, Edmar 90–92 Euwe, Max 46 Filip, Miroslav 56–59 Filip, Miroslav 10–11 Gipslis, Aivars 20–23 Gipslis, Aivars 95–96 Vlastimil Hort 54–55 Ivkov, Borislav 52 Judovič, Mihail 83–84 Karpov, Anatoly 83–84 Razuvayev, Yuri 43 Kotov, Alexander 50–51 Kotov, Alexander 06–07 Krnić, Zdenko 93–94 Krnić, Zdenko 06–07 Cvetković, Srđan 93–94 Cvetković, Srđan 85–86 Larsen, Bent 87–89 Lilienthal, Andor 87–89 Florian, Tibor 08–09 Matanović, Aleksandar 15–19 Matanović, Aleksandar 08–09 Ugrinović, Dragan 15–19 Ugrinović, Dragan 70–72 Minev, Nikolay 76–79 Minev, Nikolay 24–26 Parma, Bruno 53 Parma, Bruno 14 Polugaevsky, Lev 73–75 Polugaevsky, Lev 80–82 Razuvayev, Yuri 12–13 Suetin, Alexey 30–31 Taimanov, Mark 40–45 Taimanov, Mark 60–63 Uhlmann, Wolfgang 38–39 Unzicker, Wolfgang 64–66 Wade, Robert

The second edition was published in the following years:

• Volume A: 1996
• Volume B: 1984
• Volume C: 1981
• Volume D: 1987
• Volume E: 1991

Volume B (1984)

Code	Author
04–05	Bagirov, Vladimir
18–19	Bagirov, Vladimir
13–14	Botvinnik, Mikhail
13–14	Abramov, Lev
10	Filip, Miroslav
12	Filip, Miroslav
15	Filip, Miroslav
17	Filip, Miroslav
36–39	Filip, Miroslav
70–79	Geller, Efim
95–99	Gipslis, Aivars
20–21	Gufeld, Eduard
23–26	Gufeld, Eduard
02–03	Hort, Vlastimil
80–85	Kasparov, Garry
30–31	Korchnoi, Viktor
34–35	Korchnoi, Viktor
28–29	Krnić, Zdenko
87	Krnić, Zdenko
28–29	Cvetković, Srđan
00–01	Larsen, Bent
16	Larsen, Bent
60–69	Matanović, Aleksandar
60–69	Ugrinović, Dragan
06	Minev, Nikolay
11	Minev, Nikolay
57	Nunn, John
88–89	Nunn, John
07–09	Parma, Bruno
44	Polugaevsky, Lev
50–53	Polugaevsky, Lev
86	Polugaevsky, Lev
90–94	Polugaevsky, Lev
96	Polugaevsky, Lev
27	Suetin, Alexey
54–56	Suetin, Alexey
58–59	Suetin, Alexey
22	Sveshnikov, Evgeny
33	Sveshnikov, Evgeny
40–43	Taimanov, Mark
45–49	Taimanov, Mark
32	Uhlmann, Wolfgang

The third edition was published in the following years:

• Volume A: 1999
• Volume B: 1997
• Volume C: 1997
• Volume D: 1998
• Volume E: 1998

The fourth edition was published in the following years:

• Volume A: 2001
• Volume B: 2002
• Volume C: 2000
• Volume D: 2004
• Volume E: 2008

The fifth edition was published in the following years:

• Volume C: 2006
• Volume B part 1 [ECO codes: B00–B49]: 2020
• Volume B part 2 [ECO codes: B50-B99]: 2021

• Chess annotation symbols – ECO uses symbols instead of text
• Chess Informant
• List of chess books
• List of chess openings – for all ECO codes
• Opening book

Bibliography

• Matanović, Aleksandar; Molorović, Milivoje; Božić, Aleksandar (1971). "Classification of Chess Openings". Chess Informant. {{cite journal}}: Cite journal requires |journal= (help)
• Abramov, Lev; Bagirov, Vladimir; Botvinnik, Mikhail; Cvetkovic, Srdan; Filip, Miroslav; Geller, Efim; Gipslis, Aivars; Gufeld, Eduard; Hort, Vlastimil; Kasparov, Garry; Korchnoi, Viktor; Krnic, Zdenko; Larsen, Bent; Matanović, Aleksandar; Minev, Nikolay; Nunn, John; Parma, Bruno; Polugaevsky, Lev; Suetin, Alexey; Sveshnikov, Evgeny; Taimanov, Mark; Ugrinovic, Dragan; Uhlmann, Wolfgang (1984). Encyclopedia of Chess Openings, volume B (2nd ed.). Belgrade: Chess Informant. ISBN 0-7134-3716-2.
• Hooper, David; Whyld, Kenneth (1996) [First pub. 1992]. "Encyclopedia of Chess Openings". The Oxford Companion to Chess (2nd ed.). Oxford University Press. p. 123. ISBN 0-19-280049-3.

• "Chess Informant". Publisher's site
• "Openings". Chess Archaeology. ECO codes with names