An adpositional phrase is a syntactic category that includes prepositional phrases, postpositional phrases, and circumpositional phrases. Adpositional phrases contain an adposition (preposition, postposition, or circumposition) as head and usually a complement such as a noun phrase. Language syntax treats adpositional phrases as units that act as arguments or adjuncts. Prepositional and postpositional phrases differ by the order of the words used. Languages that are primarily head-initial such as English predominantly use prepositional phrases whereas head-final languages predominantly employ postpositional phrases. Many languages have both types, as well as circumpositional phrases.

There are three types of adpositional phrases: prepositional phrases, postpositional phrases, and circumpositional phrases.

The underlined phrases in the following sentences are examples of prepositional phrases in English. The prepositions are in bold:

Prepositional phrases have a preposition as the central element of the phrase, i.e. as the head of the phrase. The remaining part of the phrase is called the prepositional complement, or sometimes the "object" of the preposition. In English and many other Indo-European languages it takes the form of a noun phrase, such as a noun, pronoun, or gerund, possibly with one or more modifiers.

A prepositional phrase can function as an adjective or adverb.

Postpositional elements are frequent in head-final languages such as Basque, Estonian, Finnish, Georgian, Korean, Japanese, Hindi, Urdu, Bengali and Tamil. The word or other morpheme that corresponds to an English preposition occurs after its complement, hence the name postposition. The following examples are from Japanese, where the case markers perform a role similar to that of adpositions:

And from Finnish, where the case endings perform a role similar to that of adpositions:

While English is generally seen as lacking postpositions entirely, there are a couple of words that one can in fact view as postpositions, e.g. the crisis two years ago, sleep the whole night through. Since a phrase like two years ago distributes just like a prepositional phrase, one can argue that ago should be classified as a postposition, as opposed to as an adjective or adverb.