In linguistics, coordination is a complex syntactic structure that links together two or more elements; these elements are called conjuncts or conjoins. The presence of coordination is often signaled by the appearance of a coordinator (coordinating conjunction), e.g. and, or, but (in English). The totality of coordinator(s) and conjuncts forming an instance of coordination is called a coordinate structure. The unique properties of coordinate structures have motivated theoretical syntax to draw a broad distinction between coordination and subordination. It is also one of the many constituency tests in linguistics. Coordination is one of the most studied fields in theoretical syntax, but despite decades of intensive examination, theoretical accounts differ significantly and there is no consensus on the best analysis.

A coordinator or a coordinating conjunction, often appears between the conjuncts, usually at least between the penultimate and ultimate conjunct of the coordinate structure. The words and and or are by far the most frequently occurring coordinators in English. Other coordinators occur less often and have unique properties, e.g. but, as well as, then, etc. The coordinator usually serves to link the conjuncts and indicate the presence of a coordinate structure. Depending on the number of coordinators used, coordinate structures can be classified as syndetic, asyndetic, or polysyndetic.

Different types of coordinators are also categorised differently. The table below shows the categories for the coordinators in English:

Coordination is a very flexible mechanism of syntax. Any given lexical or phrasal category can be coordinated. The examples throughout this article employ the convention whereby the conjuncts of coordinate structures are marked using square brackets and bold script. In the following examples, the coordinate structure includes all the material that follows the left-most square bracket and precedes the right-most square bracket. The coordinator appears in normal script between the conjuncts.

Data of this sort could easily be expanded to include every lexical and phrasal category. An important aspect of the above data is that the conjuncts each time are constituents. In other words, the material enclosed in brackets would qualify as a constituent in both phrase structure grammars and dependency grammars.

Theoretical accounts of coordination vary in major respects. For instance, approaches to coordination in constituency and dependency differ significantly, and derivational and representational systems are also likely to disagree on many aspects of how coordination should be explained. Derivational accounts, for instance, are more likely to assume transformational mechanisms to "rectify" non-constituent conjuncts (e.g. conjunction reduction and right node raising, as mentioned below).

Even concerning the hierarchical structure of coordinated strings, there is much disagreement. Whether or not coordinate structures should be analyzed in terms of the basic tree conventions employed for subordination is an issue that divides experts. Broadly speaking, there are two options: either a flat or a layered analysis. There are two possibilities for the flat option, both of which are shown here. The a-trees represent the analyses in a constituency-based system, and the b-trees in a dependency-based system:

The first two trees present the traditional exocentric analysis. The coordinate structure is deemed exocentric insofar as neither conjunct can be taken to be the sole head, but rather both conjuncts are deemed heads in a sense. The second two trees, where the coordinator is the head, are similar to the first two insofar as the conjuncts are equi-level sisters. These two flat analyses stand in contrast to the following three layered analyses. The constituency-based a-trees appear again on the left, and the dependency-based b-trees on the right: