In grammar, the voice (or diathesis) of a verb describes the relationship between the action (or state) that the verb expresses and the participants identified by its arguments (subject, object, etc.). When the subject is the agent or doer of the action, the verb is in the active voice. When the subject is the patient, target or undergoer of the action, the verb is said to be in the passive voice. When the subject both performs and receives the action expressed by the verb, the verb is in the middle voice.

The following pair of examples illustrates the contrast between active and passive voice in English. In sentence (1), the verb form ate is in the active voice, but in sentence (2), the verb form was eaten is in the passive voice. Independent of voice, the cat is the Agent (the doer) of the action of eating in both sentences.

In a transformation from an active-voice clause to an equivalent passive-voice construction, the subject and the direct object switch grammatical roles. The direct object gets promoted to subject, and the subject demoted to an (optional) adjunct. In the first example above, the mouse serves as the direct object in the active-voice version, but becomes the subject in the passive version. The subject of the active-voice version, the cat, becomes part of a prepositional phrase in the passive version of the sentence, and can be left out entirely; The mouse was eaten.

In the grammar of Ancient Greek, voice was called διάθεσις diáthesis 'arrangement' or 'condition', with three subcategories:

In Latin, two voices were recognized:

The active voice is the most commonly used in many languages and represents the "normal" case, in which the subject of the verb is the agent. In the active voice, the subject of the sentence performs the action or causes the happening denoted by the verb. Sentence (1) is in active voice, as indicated by the verb form saw.

(1) Roger Bigod saw the castles.

The passive voice is employed in a clause whose subject expresses the theme or patient of the verb. That is, it undergoes an action or has its state changed. In the passive voice, the grammatical subject of the verb is the recipient (not the doer) of the action denoted by the verb. In English it serves a variety of functions including focusing on the object, demoting the subject and handling situations where the speaker either wants to suppress information about who the doer of the action is, or in reality does not know their identity, or when the doer is either unimportant or likely to be common knowledge. There are syntactic, semantic, and pragmatic motivations for choosing the passive voice instead of the active. Some languages, such as English and Spanish, use a periphrastic passive voice; that is, it is not a single word form, but rather a construction making use of other word forms. Specifically, it is made up of a form of the auxiliary verb to be and a past participle of the main verb which carries the lexical content of the predicate. In other languages, such as Latin, the passive voice for some tenses is simply marked on the verb by inflection: librum legit "He reads the book"; liber legitur "The book is read".