H-dropping or aitch-dropping is the deletion of the voiceless glottal fricative or "H-sound", [h]. The phenomenon is common in many dialects of English, and is also found in certain other languages, either as a purely historical development or as a contemporary difference between dialects. Although common in most regions of England and in some other English-speaking countries, and linguistically speaking a neutral evolution in languages, H-dropping is often stigmatized as a sign of careless or uneducated speech, due to its strong association with the lower class.

The reverse phenomenon, H-insertion or H-adding, is found in certain situations, sometimes as an allophone or hypercorrection by H-dropping speakers, and sometimes as a spelling pronunciation or out of perceived etymological correctness. A particular example of this is the spread of 'haitch' for 'aitch'.

In Old English phonology, the sounds [h], [x], and [ç] (described respectively as glottal, velar and palatal voiceless fricatives) are taken to be allophones of a single phoneme /h/. This phoneme occurred at the start of syllables, alone or clustered with an approximant, and in coda position. The [h] sound appeared in most onsets (except those with an /h/ and /w/ cluster, which had [x]) and the other two allophones in syllable codas ([x] after back vowels and [ç] after front vowels).

The instances of /h/ in coda position were lost during the Middle English and Early Modern English periods, although they are still reflected in the spelling of words such as taught (now pronounced like taut) and weight (now pronounced in most accents like wait). Most of the initial clusters involving /h/ also disappeared (see H-cluster reductions). As a result, in the standard varieties of Modern English, the only position in which /h/ can occur is at the start of a syllable, either alone (as in hat, house, behind, etc.), in the cluster /hj/ (as in huge), or (for a minority of speakers) in the cluster /hw/ (as in whine if pronounced differently from wine). The usual realizations of the latter two clusters are [ç] and [ʍ] (see English phonology).

The phenomenon of H-dropping considered as a feature of contemporary English is the omission, in certain accents and dialects, of this syllable-initial /h/, either alone or in the cluster /hj/. (For the cluster /hw/ and its reduction, see Pronunciation of English ⟨wh⟩.)

H-dropping, in certain accents and dialects of Modern English, causes words like harm, heat, home and behind to be pronounced arm, eat, ome and be-ind (though in some dialects an [h] may appear in behind to prevent hiatus – see below).

Cases of H-dropping occur in all English dialects in the weak forms of function words like he, him, her, his, had, and have. The pronoun it is a product of historical H-dropping – the older hit survives as an emphatic form in a few dialects such as Southern American English, and in the Scots language. Because the /h/ of unstressed have is usually dropped, the word is usually pronounced /əv/ in phrases like should have, would have, and could have. These can be spelled out in informal writing as "should've", "would've", and "could've". Because /əv/ is also the weak form of the word of, these words are often erroneously spelled as should of, would of and could of.

There is evidence of h-dropping in texts from the 13th century and later. It may originally have arisen through contact with the Norman language, where h-dropping also occurred. Puns which rely on the possible omission of the /h/ sound can be found in works by William Shakespeare and in other Elizabethan era dramas. It is suggested that the phenomenon probably spread from the middle to the lower orders of society, first taking hold in urban centers. It started to become stigmatized, being seen as a sign of poor education, in the 16th or 17th century.