Liturgy is the customary public worship done by a specific religious group, according to its particular traditions.

The word, sometimes rendered by its English translation "service", may refer to an elaborate formal ritual such as the Eastern Orthodox Divine Liturgy and Catholic Mass, or a daily activity such as the Muslim salat and Jewish services. As a religious phenomenon, liturgy is a communal response to the sacred through activity reflecting praise, thanksgiving, supplication, or repentance. Ritualization may be associated with life events such as birth, coming of age, marriage and death. It thus forms the basis for establishing a relationship with a divine agency, as well as with other participants in the liturgy. Methods of dress, preparation of food, application of cosmetics or other hygienic practices are all considered liturgical activities.

The word liturgy, derived from the technical term in ancient Greek, leitourgia, signifies the often expensive offers of service to the people, and thus to the polis and the state. Through the leitourgia the rich carried a financial burden and were correspondingly rewarded with honours. The leitourgia became both mandatory and honorific, supporting the patron's standing among the elite. The holder of a Hellenic leitourgia was not taxed a specific sum, but entrusted with a particular ritual, which could be performed with greater or lesser magnificence. The chief sphere remained that of civic religion, embodied in the festivals: M.

I. Finley notes "in Demosthenes' day there were at least 97 liturgical appointments in Athens for the festivals, rising to 118 in a (quadrennial) Panathenaic year." Eventually, under the Roman Empire, such obligations, known as munera devolved into a competitive and ruinously expensive burden that was avoided when possible.