Thai classical music is synonymous with those stylized court ensembles and repertoires that first emerged within the royal centers of Central Thailand some 800 years ago. These ensembles, while being deeply influenced by Khmer and even older practices and repertoires from India, are today uniquely Thai expressions. While the three primary classical ensembles, the piphat, khruang sai and mahori differ in significant ways, they all share a basic instrumentation and theoretical approach. Each employ the small ching hand cymbals and the krap wooden sticks to mark the primary beat reference. Several kinds of small drums (klong) are employed in these ensembles to outline the basic rhythmic structure (natab) that is punctuated at the end by the striking of a suspended gong (mong). Seen in its most basic formulation, the classical Thai orchestras are very similar to the Cambodian (Khmer) pin peat and mahori ensembles, and structurally similar to other orchestras found within the wide-spread Southeast Asian gong-chime musical culture, such as the large gamelan of Bali and Java, which most likely have their common roots in the diffusion of Vietnamese Dong-Son bronze drums beginning in the first century ACE.

Traditional Thai classical repertoire is anonymous, handed down through an oral tradition of performance in which the names of composers (if, indeed, pieces were historically created by single authors) are not known. However, since the beginning of the modern Bangkok period, composers' names have been known and, since around the turn of the century, many major composers have recorded their works in notation. Musicians, however, imagine these compositions and notations as generic forms which are realized in full in idiosyncratic variations and improvisations in the context of performance. While the composer Luang Pradit Phairau (1881–1954) used localized forms of cipher (number) notation, other composers such as Montri Tramote (1908–1995) used standard western staff notation. Several members of the Thai royal family have been deeply involved in composition, including King Prajatipok (Rama VII, 1883–1941) and King Bhumibol Adulyadej (1927–), whose compositions have been more often for jazz bands than classical Thai ensembles.

The most common and iconic Thai classical ensemble is the piphat, a midsized orchestra including two xylophones (ranat), an oboe (pinai), barrel drums (klong) and two circular sets of tuned horizontal gong-chimes (kong wong). Piphat can be performed in either a loud outdoor style using hard mallets or in an indoor style using padded hammers. There are several types of piphat ensembles ranging in size and orchestration, each kind typically being associated with specific ceremonial purposes. The highly decorated piphat ensemble that features the ornately carved and painted semicircular vertical gong-chime is traditionally associated with the funeral and cremation ceremonies of the Mon ethnic group. Different versions of the piphat ensemble are employed to accompany specific forms of traditional Thai drama such as the large shadow puppet theater (nang yai) and the khon dance drama

The khruang sai orchestra combines some of the percussion and wind instruments of the piphat with an expanded string section including the so duang (a high-pitched two-string bowed lute), the lower pitched solaw (bowed lute) and the three-string jhakhe (a plucked zither). In addition to these instruments are the klhui (vertical fipple flute) in several sizes and ranges, a goblet drum (than) and, occasionally, a small hammered Chinese dulcimer (khim). The khruang sai ensemble is primarily used for instrumental indoor performances and for accompanying the Thai hoon grabok (stick-puppet theater), a genre deeply influenced by Chinese puppetry styles. Accordingly, the addition of Chinese-sounding string instruments in the khruang sai ensemble is imagined, by the Thai, to be a reference to the probable Chinese origins of this theater form.

The third major Thai classical ensemble is the mahori, traditionally played by women in the courts of both Central Thailand and Cambodia. Historically the ensemble included smaller instruments more appropriate, it was thought, to the build of female performers. Today the ensemble employs regular sized instruments—a combination of instruments from both the khruang sai and piphat ensembles but excluding the loud and rather shrill oboe. The ensemble, which is performed in three sizes—small, medium and large—includes the three-string so sam sai fiddle, a delicate-sounding, middle-range bowed lute with silk strings. Within the context of the mahori ensemble, the so sam sai accompanies the vocalist, which plays a more prominent role in this ensemble than in any other classical Thai orchestra.

While Thai classical music was somewhat discouraged as being unmodern and backward looking during Thailand's aggressively nationalistic modernization policies of mid-20th century, the classical arts have benefited recently from increased governmental sponsorship and funding as well as popular interest as expressed in such films as Homrong (2003), a popular fictionalized biography of a famous traditional xylophone (ranat) performer. —Andrew McGraw