The laryngeal theory is a theory in the historical linguistics of the Indo-European languages positing that:

• The Proto-Indo-European language (PIE) had a series of phonemes beyond those reconstructable by the comparative method. That is, the theory maintains that there were sounds in Proto-Indo-European that no longer exist in any of the daughter languages, and thus, cannot be reconstructed merely by comparing sounds among those daughter languages.
• These phonemes, according to the most accepted variant of the theory, were laryngeal consonants of an indeterminate place of articulation towards the back of the mouth.

The theory aims to:

• Produce greater regularity in the reconstruction of PIE phonology than from the reconstruction that is produced by the comparative method.
• Extend the general occurrence of the Indo-European ablaut to syllables with reconstructed vowel phonemes other than *e or *o.

In its earlier form (see below), the theory proposed two sounds in PIE. Combined with a reconstructed *e or *o, the sounds produce vowel phonemes that would not otherwise be predicted by the rules of ablaut. The theory received considerable support after the deciphering of Hittite, which revealed it to be an Indo-European language.

Many Hittite words were shown to be derived from PIE, with a phoneme represented as ḫ corresponding to one of the hypothetical PIE sounds. Subsequent scholarly work has established a set of rules by which an ever-increasing number of reflexes in daughter languages may be derived from PIE roots. The number of explanations thus achieved and the simplicity of the postulated system have both led to widespread acceptance of the theory.

In its most widely accepted version, the theory posits three laryngeal phonemes in PIE: h₁, h₂, and h₃ (see below). Daughter languages other than Hittite did not preserve the laryngeals themselves, but inherited sounds derived from the merger of these laryngeals with PIE short vowels and the subsequent loss of those laryngeals.

The phonemes are now recognized as consonants, related to articulation in the general area of the larynx, where a consonantal gesture may affect vowel quality. They are regularly known as laryngeal, but the actual place of articulation for each consonant remains a matter of debate. (see below).

The laryngeals got their name because they were believed by Hermann Möller and Albert Cuny to have had a pharyngeal, epiglottal, or glottal place of articulation, involving a constriction near the larynx. While this is still possible, many linguists now think of laryngeals, or some of them, as having been velar or uvular.

The evidence for their existence is mostly indirect, as will be shown below, but the theory serves as an elegant explanation for several properties of the PIE vowel system that made no sense until the theory, such as the independent schwas (as in *pəter- 'father'). Also, the hypothesis that PIE schwa *ə was a consonant, not a vowel, provides an elegant explanation for some apparent exceptions to Brugmann's law in Indo-Aryan languages.