The Encyclopaedia of Chess Openings (ECO) is a classification system for the opening moves in a game of chess. It is presented as a five volume book collection (now also a computer database) describing chess openings. The moves were taken from hundreds of thousands of games between masters, from published analysis in the Chess Informant since 1966, and then compiled by notable chess players. The main editor is Aleksandar Matanovic. Both the ECO and the Chess Informant are published by the Serbian company Sahovski informator (Chess Informant). These openings are typically provided in an ECO table that concisely presents the best opening lines.

The book contain only a small amount of text, which is in eight languages. The bulk of the book consists of diagrams of positions and chess moves in International figurine algebraic notation, with special symbols for comments about moves.

Instead of the traditional names for the openings, ECO has developed a unique coding system that has also been adopted by other chess publications. There are five main categories, “A” to “E”, each of which is divided into one hundred sub-categories.