国际象棋的最初阶段称为「开局」。刚开始的几个回合棋手主要关心自己的棋子是否从起始点移动到有利的位置。游戏的初期，白方发动的一连串经过检验的步法称为「开局」，而黑方回应的步法称为「防御」。为人熟知的开局走法有好几十种，而其中又有好几百种被赋予名称的变着。《牛津国际象棋大辞典》列出了1,327有名称的开局与变着，从平和的局面型走法，到激烈的战术型走法通通包括在内。

一连串正规的步法称为「开局步法」，一些工具书如《国际象棋开局百科》有对开局步法做了编目索引。这些工具书通常利用代数记谱法、开局树状图、或开局理论表将一连串的步法呈现给读者。如果开局偏离了「开局步法」，之后的变化就称为「out of book」。某些开局的主线已研究到20到25步之多。有一些甚至还分析到了第30或35步，像是古印度防御、西西里防御中的舍文宁根变着、以及纳依道夫变着。职业棋手们花费数年的时间来研究开局，而随着开局理论不断演进，许多棋手甚至终其一生都在研究开局。

A chess opening is the group of initial moves of a chess game. Recognized sequences of initial moves are referred to as openings by White, or defenses by Black, but opening is also used as the general term. There are many dozens of different openings, and hundreds of named variants. The Oxford Companion to Chess lists 1,327 named openings and variants. These vary widely in character from quiet positional play to wild tactical play. In addition to referring to specific move sequences, the opening is the first phase of a chess game, the other phases being the middlegame and the endgame.