Labialization is a secondary articulatory feature of sounds in some languages. Labialized sounds involve the lips while the remainder of the oral cavity produces another sound. The term is normally restricted to consonants. When vowels involve the lips, they are called rounded.

The most common labialized consonants are labialized velars. Most other labialized sounds also have simultaneous velarization, and the process may then be more precisely called labio-velarization. The "labialization" of bilabial consonants often refers to protrusion instead of a secondary articulatory feature velarization. [pʷ] doesn't mean [pˠ] although [w] refers to a labial–velar approximant.

In phonology, labialization may also refer to a type of assimilation process.

Labialization is the most widespread secondary articulation in the world's languages. It is phonemically contrastive in Northwest Caucasian (e.g. Adyghe), Athabaskan, and Salishan language families, among others. This contrast is reconstructed also for Proto-Indo-European, the common ancestor of the Indo-European languages; and it survives in Latin and some Romance languages. It is also found in the Cushitic and Ethio-Semitic languages.

American English labializes /r, ʃ, ʒ, tʃ, dʒ/ to various degrees.

A few languages, including Arrernte and Mba, have contrastive labialized forms for almost all of their consonants.

In many Salishan languages, such as Klallam, velar consonants only occur in their labialized forms (except /k/, which occurs in some loanwords). However, uvular consonants occur abundantly labialized and unrounded.

Out of 706 language inventories surveyed by Ruhlen (1976), labialization occurred most often with velar (42%) and uvular (15%) segments and least often with dental and alveolar segments. With non-dorsal consonants, labialization may include velarization as well. Labialization is not restricted to lip-rounding. The following articulations have either been described as labialization or been found as allophonic realizations of prototypical labialization:

• Labiodental frication, found in Abkhaz^[1]
• Complete bilabial closure, [d͡b, t͡p, t͡pʼ], found in Abkhaz and Ubykh^[1]
• "Labialization" (/w/, /ɡʷ/, and /kʷ/) without noticeable rounding (protrusion) of the lips, found in the Iroquoian languages^{[citation needed]}. It may be that they are compressed.^{[citation needed]}
• Rounding without velarization, found in Shona^{[citation needed]} and in the Bzyb dialect of Abkhaz.^{[citation needed]}

Eastern Arrernte has labialization at all places and manners of articulation; this derives historically from adjacent rounded vowels, as is also the case of the Northwest Caucasian languages. Marshallese also has phonemic labialization as a secondary articulation at all places of articulation except for labial consonants and coronal obstruents.

In North America, languages from a number of families have sounds that sound labialized (and vowels that sound rounded) without the participation of the lips. See Tillamook language for an example.

In the International Phonetic Alphabet, labialization of velar consonants is indicated with a raised w modifier [ʷ] (Unicode U+02B7), as in /kʷ/. (Elsewhere this diacritic generally indicates simultaneous labialization and velarization.^{[citation needed]}) There are also diacritics, respectively [ɔ̹], [ɔ̜], to indicate greater or lesser degrees of rounding.^[3] These are normally used with vowels but may occur with consonants. For example, in the Athabaskan language Hupa, voiceless velar fricatives distinguish three degrees of labialization, transcribed either /x/, /x̹/, /xʷ/ or /x/, /x̜ʷ/, /xʷ/.

The extensions to the IPA has two additional symbols for degrees of rounding: Spread [ɹ͍] and open-rounded [ʒꟹ] (as in English). It also has a symbol for labiodentalized sounds, [tᶹ].^[4]

If precision is desired, the Abkhaz and Ubykh articulations may be transcribed with the appropriate fricative or trill raised as a diacritic: [tᵛ], [tᵝ], [t^ʙ], [tᵖ].

For simple labialization, Ladefoged & Maddieson (1996) resurrected an old IPA symbol, [ ̫],^[5] which would be placed above a letter with a descender such as ɡ. However, their chief example is Shona sv and zv, which they transcribe /s̫/ and /z̫/ but which actually seem to be whistled sibilants, without necessarily being labialized.^[6] Another possibility is to use the IPA diacritic for rounding, distinguishing for example the labialization in English soon [s̹] and [sʷ] swoon.^[7] The open rounding of English /ʃ/ is also unvelarized.

Labialization also refers to a specific type of assimilatory process where a given sound become labialized due to the influence of neighboring labial sounds. For example, /k/ may become /kʷ/ in the environment of /o/, or /a/ may become /o/ in the environment of /p/ or /kʷ/.

In the Northwest Caucasian languages as well as some Australian languages rounding has shifted from the vowels to the consonants, producing a wide range of labialized consonants and leaving in some cases only two phonemic vowels. This appears to have been the case in Ubykh and Eastern Arrernte, for example. The labial vowel sounds usually still remain, but only as allophones next to the now-labial consonant sounds.

• Labio-palatalization (◌ᶣ)