In poetry, metre (Commonwealth spelling) or meter (American spelling; see spelling differences) is the basic rhythmic structure of a verse or lines in verse. Many traditional verse forms prescribe a specific verse metre, or a certain set of metres alternating in a particular order. The study and the actual use of metres and forms of versification are both known as prosody. (Within linguistics, "prosody" is used in a more general sense that includes not only poetic metre but also the rhythmic aspects of prose, whether formal or informal, that vary from language to language, and sometimes between poetic traditions.)

An assortment of features can be identified when classifying poetry and its metre.

The metre of most poetry of the Western world and elsewhere is based on patterns of syllables of particular types. The familiar type of metre in English-language poetry is called qualitative metre, with stressed syllables coming at regular intervals (e.g. in iambic pentameters, usually every even-numbered syllable). Many Romance languages use a scheme that is somewhat similar but where the position of only one particular stressed syllable (e.g. the last) needs to be fixed. The alliterative metre of the old Germanic poetry of languages such as Old Norse and Old English was radically different, but was still based on stress patterns.

Some classical languages, in contrast, used a different scheme known as quantitative metre, where patterns were based on syllable weight rather than stress. In the dactylic hexameters of Classical Latin and Classical Greek, for example, each of the six feet making up the line was either a dactyl (long-short-short) or a spondee (long-long): a "long syllable" was literally one that took longer to pronounce than a short syllable: specifically, a syllable consisting of a long vowel or diphthong or followed by two consonants. The stress pattern of the words made no difference to the metre. A number of other ancient languages also used quantitative metre, such as Sanskrit, Persian, Old Church Slavonic and Classical Arabic (but not Biblical Hebrew).

Finally, non-stressed languages that have little or no differentiation of syllable length, such as French or Chinese, base their verses on the number of syllables only. The most common form in French is the Alexandrin, with twelve syllables a verse, and in classical Chinese five characters, and thus five syllables. But since each Chinese character is pronounced using one syllable in a certain tone, classical Chinese poetry also had more strictly defined rules, such as thematic parallelism or tonal antithesis between lines.

In many Western classical poetic traditions, the metre of a verse can be described as a sequence of feet, each foot being a specific sequence of syllable types – such as relatively unstressed/stressed (the norm for English poetry) or long/short (as in most classical Latin and Greek poetry).

Iambic pentameter, a common metre in English poetry, is based on a sequence of five iambic feet or iambs, each consisting of a relatively unstressed syllable (here represented with "˘" above the syllable) followed by a relatively stressed one (here represented with "/" above the syllable) – "da-DUM"="˘ /":

This approach to analyzing and classifying metres originates from Ancient Greek tragedians and poets such as Homer, Pindar, Hesiod, and Sappho.