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 Serbian 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.

The first edition was published in the following years:

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

The second edition was published in the following years:

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

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