The game of chess is commonly divided into three phases: the opening, middlegame, and endgame. There is a large body of theory regarding how the game should be played in each of these phases, especially the opening and endgame. Those who write about chess theory, who are often also eminent players, are referred to as "theorists" or "theoreticians".

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. It is currently undergoing its fifth edition. ECO may also refer to the opening classification system used by the encyclopedia.