Most Hmong-Mien speakers belong to the Miao and Yao (Mien) nationalities, two minority ethnic groups in China, although not all Miao or Yao people speak a Hmong-Mien language. Hmong-Mien speakers in China primarily inhabit Guizhou, Hunan, and Yunnan provinces and the Zhuang Autonomous Region of Guangxi, although smaller numbers live in Sichuan, Guangdong, Hubei, and Jiangxi provinces and on the island of Hainan.

Under pressure from the dominant Han Chinese population, waves of Hmong and Mien speakers migrated to Southeast Asia during the 19th and early 20th centuries. Another wave of migration followed the end of the Vietnam War in the 1970s, when tens of thousands of Southeast Asian Hmong and Mien emigrated to the United States, France, French Guiana, and Australia. At the beginning of the 21st century, the total number of Hmong-Mien speakers worldwide was estimated to be approximately 10 million. However, as six out of seven speakers of Hmong-Mien languages live in China, and the Chinese government reports the number of people in the Miao and Yao ethnic groups (which may contain speakers of languages other than Hmong-Mien), the actual number may be somewhat smaller.

Most Chinese scholars have claimed that Hmong-Mien belongs to the Sino-Tibetan (Sino-Tibetan languages) linguistic family, along with Chinese (Chinese languages), Tibeto-Burman (which includes Tibetan, Burmese, Karen, and many smaller languages of southern and western Asia), and Tai-Kadai (which includes Thai, Lao, Shan, Zhuang, and many smaller languages of Southeast Asia). Although a genetic relationship between Chinese and Tibeto-Burman is generally accepted, the belief that this family also includes Hmong-Mien and Tai-Kadai is not widely shared by linguists outside China. Despite massive numbers of Chinese loanwords in Hmong-Mien languages, an examination of recurrent sound correspondences in basic vocabulary, the methodology upon which the determination of genetic affiliation has rested since the 19th century, does not support the theory that Chinese and Hmong-Mien are related. Furthermore, linguists outside China reject similarities in grammar, word structure, and phonological systems as evidence of genetic relationship; these similarities have been ascribed to the power of Chinese influence in the area and widespread bilingualism.

Other possible family connections have been proposed. In 1948 the English linguist R.A.D. Forrest accepted and elaborated on Henry R. Davies's hypothesis (1909) of a link between Hmong-Mien and Mon-Khmer. In 1975 the American linguist Paul K. Benedict linked Hmong-Mien to Austronesian and Tai-Kadai as part of a family he had labeled “Austro-Tai” in earlier work. Although the Austric hypothesis, first proposed by the German linguist Wilhelm Schmidt, originally linked only Austroasiatic and Austronesian, Hmong-Mien has been mentioned as a possible member of this constellation as well. None of these proposals has gained general acceptance among scholars. Until a careful separation of layers of Chinese borrowings from native Hmong-Mien vocabulary has been completed, the question of wider family connections cannot be resolved. The most prudent position to take in the meantime is that Hmong-Mien constitutes an independent family of languages.

Within the family two main branches have been identified: the Hmongic and the Mienic. The Hmongic (Miao) subfamily is an internally diverse group that includes mutually unintelligible languages such as Hmu (spoken in Guizhou and Guangxi), Hmong (spoken in Guizhou and Yunnan and in Southeast Asia), Qo Xiong (spoken in Hunan), Bunu (spoken in Guangxi), and Ho Ne (also known as She; spoken in Guangdong). The Mienic (Yao) subfamily is smaller and less diverse but is scattered over the entire geographical area. It includes the languages Iu Mien, Mun, and Biao Min, among others. Further research on lesser-known members of the family may lead to the refinement of this simple family tree.

The designation Miao-Yao, the name of these ethnic groups and an alternate name for the language family, is of Chinese origin. It represents the concept of “nationality,” which is not a purely linguistic classification but also takes into account culture, politics, and self-identification. For example, speakers of Mien together with speakers of Bunu, a Hmongic language, and Lakkia, a Tai-Kadai language, are classified in China as members of the Yao nationality. Conversely, for cultural reasons, speakers of the Mun language on Hainan Island are classified in China as members of the Miao nationality despite the fact that their language is Mienic. To avoid this confusion of ethnic and linguistic categories, many Western scholars have adopted the name Hmong-Mien to refer to this language family.

Most words in Hmong-Mien languages are monosyllabic. For the vast majority of words, therefore, word structure is the same as syllable structure: CV(C)—i.e., an initial consonant or consonant cluster followed by a vowel, in some cases closed by a final consonant. Each word also bears a distinctive tone, which contributes as much to the identification of the word as do the consonants and vowels.

All Hmong-Mien languages are tonal languages, some with large and complex inventories of tones. For example, five of the eight Shidongkou Hmu (“Black Miao”) tones are level tones that are spoken at different pitches, a world-record number of pitch level distinctions. Longmo Bunu and Zongdi Hmong each have 12 tonal contrasts including both level and contour tones (tones which change pitch), in comparison with the four tones of Mandarin Chinese (Chinese languages). Tones in some Hmong-Mien languages are characterized by voice quality distinctions as well as pitch contrasts: it is not unusual for certain tones to be “breathy” or “whispered” in quality, and occasionally a tone may be characterized by a “creaky” voice quality (which can be approximated by lowering the pitch of one's voice until it begins to break).

Words in Hmongic languages have a wide variety of initial consonants (consonant). Consonants are articulated at several different places in the mouth, including familiar sounds made at the front of the mouth with the lips (labial consonants), with the tongue touching or near the teeth (dental/alveolar consonants), and with the back of the tongue touching the soft palate (velar consonants). These three types of consonants also contrast in English /p/, /t/, and /k/. However, Hmongic languages also have consonant articulations that are not found in English, including a retroflex series, in which the tip of the tongue curls backward to contact the gum ridge behind the upper teeth, and a uvular series, in which the back of the body of the tongue raises to contact the soft palate at a point behind the contact point for a /k/. Additionally, stop consonants characterized by a complete blockage of the airflow from the lungs come in different varieties. At each place of articulation, a stop consonant may be produced with or without voicing (vocal fold vibration), as in the voiceless /p/ in English pea versus the voiced /b/ in English bee, although voiced stops are less common than voiceless stops across the family. Stops may also be produced with or without distinctive aspiration (a puff of air), as in the aspirated /p^h/ in English pot versus the unaspirated /p/ in English spot. Stops may be prenasalized as well. This yields a possible four-way contrast at each place of articulation: for example, /p/, /b/, /p^h/, and /mp/. There may also be a three-way distinction at each place of articulation for sounds that allow a small amount of air to escape, such as the fricatives (fricative) /s/, /z/, and /s^h/. Nasal consonants and /l/ often occur in pairs governed by the presence or absence of vocal fold vibration, such as /m/ and // or /l/ and //. Finally, clusters of consonants with /l/ and /j/ are common—for example, /pl/, /mpl/, /mp^hl/, /pj/, /mpj/, and /mp^hj/.

When syllables in Hmongic languages end in a consonant, they end in either /n/ or /ŋ/. Syllables in Mienic languages, on the other hand, can end in any one of six consonants: /m/, /n/, /ŋ/, /p/, /t/, or /ʔ/. The high number of possible consonants in final position seems to correspond to fewer options for initial consonants. Vowel systems in both Hmongic and Mienic languages include both simple vowels and diphthongs (diphthong); Mienic languages may have a contrast between short and long vowels as well, as in the Mien pair /dât/ ‘to weave (cloth)' and /dâat/ ‘wing.'

Speakers of Hmongic and Mienic languages have long been dominated by speakers of Chinese (Chinese languages), and their languages reflect this. There are typological similarities between members of the two language families as well as high numbers of Chinese loanwords in Hmong-Mien languages. In terms of word structure (morphology), there are no suffixes, and the only common prefixes are noun prefixes whose functions are eroded and unclear. Tones are overwhelmingly used to differentiate words; they only rarely serve a grammatical function. Number and case are not marked on the noun or pronoun, nor are tense, mood, or aspect marked on the verb. There is no agreement marking of any type between words in the clause. New words are formed by making compounds, as in modern Chinese. Reduplication or doubling of a word, usually with an intensifying effect, is also common, as in the White Hmong phrase /ntau ntau/ ‘very much,' from the adverb /ntau/ ‘much.'

In the absence of grammatical structure within the word, clause-level structure ( syntax) and the structure of units larger than the sentence (discourse) become more important. Hmong-Mien languages are characterized by a number of syntactic and discourse structures common to languages of the area, as can be seen in the following examples from the White Hmong dialect of Southeast Asia.

First, the main clause word order is SVO (subject–verb–object). The sentence “you drink water” is therefore constructed as /kô/ ‘you' + /haù/ ‘drink' + /dê/ ‘water.'

/ ‘ball.'

/ ‘he or she' + /nti/ (numeral classifier for rings) + /np^hlái/ ‘ring.'

Fourth, Hmong-Mien languages contain a separate part of speech known as an expressive or an ideophone, which refers to aesthetic aspects of the passing scene such as pattern of movement, sound, or the appearance of the object being observed. They constitute a separate part of speech since they have their own characteristic internal structure and role outside the structure of the clause. An example from White Hmong is /dhû dhŋ/, which has an abstract meaning of ‘in and out' and can be used to describe the sensation of almost remembering and then forgetting again, or rising from and falling back into unconsciousness.

Finally, sentence discourse particles, usually found at the end of utterances, express a variety of speaker attitudes, such as confirmation of or doubt about the proposition, impatience, or a call for a rejoinder from the other party in the conversation. So, for example, to say “What are you doing then?” and express exasperation, a speaker of White Hmong would say /nê/ ‘you all' + /ua/ ‘do' + /dátʃi/ ‘what' + /nâ/ (discourse particle)!

All existing writing systems for Hmong-Mien languages were developed in the 20th century. Alphabetic systems based on Roman letters have been used most often and most successfully. Chinese scholars have used the Roman alphabet to produce dictionaries and literacy materials for the Yanghao dialect of the Hmu language, the Xianjin dialect of the Hmong language, and the Jiwei dialect of the Qo Xiong language. In the 1950s missionaries from the United States and France developed romanized writing systems for the Hmong and Mien dialects spoken in Southeast Asia. All of the romanized systems take advantage of the fact that final consonants are rare: they use the final consonant symbol in a written word to represent the tone of the word rather than a final consonant sound. Although the writing systems developed in both places use Latin letters, the letters are pronounced quite differently in each system. In writing systems developed in China, the letter-to-sound relationships follow the conventions of the Pinyin romanization of Chinese. In writing systems developed in Southeast Asia, on the other hand, the letter-to-sound relationships follow the conventions of romanized Vietnamese (Vietnamese language).

Two original writing systems developed for the Hmong language have enjoyed some success. Between 1904 and 1936 the missionary Samuel Pollard invented the Pollard script for writing A-Hmao, a Hmongic language spoken in northeast Yunnan and northwest Guizhou provinces. The Pollard system uses primary symbols to represent consonants and smaller secondary symbols to represent vowels. The placement of the vowel symbols in relation to the consonant symbols (above, to the right, and so forth) indicates tone. Many literate A-Hmao speakers prefer the Pollard script to the romanized system introduced by the Chinese. A second system of writing Hmong languages was developed in 1959–71 in Laos by native leader Shong Lue Yang. He invented and developed increasingly sophisticated versions of the Pahawh Hmong writing system, which is based on the “onset” (the initial consonant or consonant cluster in a syllable) and the “rime” (the vowel or vowel cluster at the nucleus of the syllable and the final consonant).