Opening theory for the traditional Asian game of go has been studied in depth for many centuries. Despite that, the theory is very far from complete, and is subject to changes of fashion, and also some notable periods of innovation. Opening theory is less dominant in terms of study, for those wanting to reach a good amateur level, than in chess or shogi. It is, however, an important component of go knowledge; though there is no single, codified source for it.

The theory is traditionally divided into the study of sequences that are whole board openings, and those that are corner openings. The study of corner openings has been developed more intensely; it is often said that a player of professional level is able to recall 5000 different corner variations.

The Japanese word joseki (or Korean jungsuk) is often applied to such corner variations. This is potentially inaccurate, in that it means something like 'set sequence', without implying that this is confined to one corner. Corner joseki conventionally start with one player occupying a corner point, in an empty 10×10 area of the board, and the other player replying with an approach move (Japanese kakari). The initial play in the corner is almost always on a 3-3, 3-4, 3-5, 4-4 or 4-5 point.

Of those plays, the classical 3-4 point (komoku) and more contemporary 4-4 point (hoshi) are the most used. The standard approaches are at 5-3 or 5-4 to the 3-4 point, and at 3-6/6-3 to the 4-4 point. The number of subsequent variations is then quite large (of the order of ten reasonable plays for the next one).

The standard sequences for the corner plays in many cases come to a definite end, after which both players should move elsewhere. In some cases a sharp local struggle breaks out, which neither player should neglect. For those cases, the result of the opening may develop out of a 10×10 corner area into the rest of the board. Analysis without taking into account what other stones are in place then becomes somewhat meaningless. The longest 'book' corner openings are about 50-ply.

Most corner openings do not have special or picturesque names. A few that do are known by Japanese names: the taisha , the nadare (avalanche), the Magic sword of Muramasa. These are amongst the most complex, and are counterindicated for novices.

Patterns for play on the whole board have seen much less systematic study, and a game may easily explore an unfamiliar path. Again, only a relatively small proportion of openings have a recognised name. These include the Three stars opening (sanrensei ), Two stars opening (nirensei ), and Chinese opening . To be more precise these are names, rather, for the formation Black (the player who starts) adopts on one side of the board; since White has a choice of at least two dozen formations, on the other side, these are in fact large complexes of openings. The Chinese opening, which has a complex pre-history but was indeed developed by Chinese players, was very popular from about 1970 onwards, and has by go standards a thoroughly-researched theory.

Joseki

A star-point joseki: Black "O" is on the 4-4 point, or "hoshi." White approaches with 1, B2 gives Black a foothold, and this particular joseki continues as shown. White 5 ends the sequence. Black 6 is played elsewhere (tenuki). A subsequent follow-up white move at 7 threatens the black group's base: Black 8 in response might be played to make life, but in practice Black would perhaps escape to the centre first.

Strictly joseki are sequences of moves

• that have been played and documented in high-level play, and

• that have been studied and deemed as consisting of optimal moves for both sides.

Joseki is a Japanese word (定石), where 定 means "fixed", "set" and 石 means stone(s). It thus literally means "set stones", as in "set pattern". It is sometimes also used outside of Go in this sense; In everyday speech, "This is the joseki here" may apply to just about anything.

The current joseki are not a fixed body of knowledge, but patterns that have gained acceptance in professional games. That is, they form a consensus judgement, that might change in the future. A joseki may fall out of use for various reasons, some of which may often seem minor to the amateur player (e.g. "in this variation that stone is not working quite as well as in another variation, therefore this one is slightly suboptimal"). There is no definitive guide to what is joseki; the situation with joseki dictionaries is similar to that of natural language dictionaries, in that some entries are obsolescent and the listing is not likely to be complete. Fashion also to some extent plays a role.

Variations lead to different positional advantages and disadvantages for the two players, and their study—not by rote, rather by understanding the principles behind each move—is an important part developing one's strength as a player. Knowing a particular joseki simply means that one knows a sequence of moves, resulting in a balance or fair trade-off between their positions. This is in practice much easier than appraising how joseki relate to the rest of the board; joseki knowledge is shallow compared with the insight needed to play an integrated strategy.

"Balance" in this case is often a trade off between securing territory in the corner versus making good thickness toward the sides and center. But after a point, these are very dynamic, and often joseki are deviated from depending on the needs of the situation, and the opportunities available. While learning joseki is a tool to defend against a local loss, players always seek to take advantage of weaknesses in the opponent's shapes, often deviating from the joseki.

There is a go proverb that states that "learning joseki loses two stones in strength," meaning that rote learning of sequences is not advantageous. Rather learning from joseki should be a player's goal. Every joseki should be used as a specific tool that leaves the board in a particular shape. Just as using an improper tool in machinery can be devestating, choosing the wrong joseki can easily be worse than improvising one's own moves.

Also useful is the tenuki concept of breaking away from a sequence, to play elsewhere, before the 'official' endpoint of the joseki. After a joseki sequence has ended, a play returning to the same area may be termed a follow-up play. There is no formal theory for these, though numerous set sequences can be seen in professional play.