A macron (from Gr. μακρός makros "large") is a diacritic ¯ placed over a vowel originally to indicate that the vowel is long. The opposite is a breve ˘, used to indicate a short vowel. These distinctions are usually phonemic.

In modern Old English transliterations, the macron has been used in this way.

In Latvian A-macron, E-macron, I-macron and U-macron are considered separate letters that sort in alphabetical order immediately after A, E, I, U respectively. For instance, baznīca comes before bārda in a Latvian dictionary.

In pinyin, macrons are used over a, e, i, o, u, ü (ā, ē, ī, ō, ū, ǖ) to indicate the first tone of Mandarin Chinese. It does not indicate vowel length in any way.

In the International Phonetic Alphabet, macron over a vowel indicates a mid-level tone, and not length.

In Hawaiian (where it is known as the kahakō) it is again used to indicate long vowels, which in turn influence the placement of accent stress in words.

Early writing in Māori did not distinguish vowel length. Some have advocated that the double vowel orthography be used to distinguish vowel length. However, the Māori Language Commission (Te Taura Whiri) advocate a macron be used to designate a long vowel. The use of the macron is now wide spread in modern Māori writings, though many people use a diaeresis mark instead (e.g. Mäori instead of Māori) due to lack of support on computers.

It is also used in many dictionaries and textbooks to mark vowel length in languages that do not feature this diacritic in everyday use; for example it is used in the Hepburn transcription of Japanese to indicate a long vowel, as in kōtsū (交通) "traffic" as opposed to kotsu (骨) "bone" or "knack (fig.)". It is often used in modern Latin dictionaries to mark vowel length, in conjuction with the breve.

In Unicode, "combining macron" is one of the combining diacritical marks, its code is U+0304 (in HTML, ̄ or ̄). There are also several precomposed characters; their HTML/Unicode numbers are as in the table to the right.

If the last two rows of the table do not display properly, the row before the last is the letter Uu with diaeresis (Ü ü) and macron, used in pinyin. The final row is the letter Yy with macron, used sometimes in teaching Latin.