Origin of the English Term
Compline is the final church service of the day in the daily liturgical cycle, prior to going to sleep. The English word Compline is derived from the Latin completorium, as Compline is the completion of the working day. The word was first used in this sense about the beginning of the 6th century by St. Benedict in his Rule (Regula Benedicti; hereafter, RB), in Chapters 16, 17, 18, and 42, and he even uses the verb complere to signify Compline: "Omnes ergo in unum positi compleant" ("All having assembled in one place, let them say Compline"); "et exuentes a completorio" ("and, after going out from Compline...") (RB, Chap. 42).
- This section incorporates information from the Catholic Encyclopedia of 1917. References to psalms follow the numbering system of the Septuagint.
The origin of Compline has given rise to considerable discussion among liturgists. In the past, general opinion (including Bäumer and Batiffol) ascribed the origin of this Hour to St. Benedict, in the beginning of the 6th century. But Father Pargoire and, later still, A. Vandepitte oppose this opinion and seek a more ancient origin for this Hour.
A text in Callinicus (between 447 and 450), first introduced in Father Pargoire's argument, informs us that between Vespers and the Midnight Office there was celebrated in the East a canonical Hour called in this text prothypnia, because it preceded the first sleep, being nothing other than what the Greeks today call apodeipnon, on account of the meal it follows. However, in the thirty-seventh question of his Great Asketikon (Long Rules), St. Basil the Great, also, speaks of an intermediate Hour between Vespers and the Midnight Office. Father Pargoire therefore disputes the assertion that St. Benedict was the originator of Compline, being rather disposed to trace its source to St. Basil.
In the article mentioned above, Father Vandepitte confirms these conclusions; nevertheless he states, in the clearest terms, that it was not in Cæsarea in 375, but in his retreat in Pontus (358-362), that Basil established Compline, which Hour did not exist prior to his time, that is, until shortly after the middle of the 4th century. Dom Plaine also traced the source of Compline back to the 4th century, finding mention of it in a passage in Eusebius and in another in St. Ambrose, and also in John Cassian. These passages have been critically examined, and Fathers Pargoire and Vandepitte have proved that before St. Basil's time the custom of reciting Compline was unknown.
At any rate, even if these texts do not express all that Dom Plaine says they do, at least they bear witness to the private custom of saying a prayer before retiring to rest. If this was not the canonical Hour of Compline, it was certainly a preliminary step towards it.
The same writers reject the opinion of Ladeuze and Dom Besse, both of whom believe that Compline had a place in the Rule of St. Pachomius, which would mean that it originated still earlier in the 4th century.
It is not necessary to enter into this discussion, but it might be possible to conciliate these different sentiments by stating that, if it be an established fact that St. Basil instituted and organized the Hour of Compline for the East, as St. Benedict did for the West, there existed as early as the days of St. Cyprian and Clement of Alexandria the custom of reciting a prayer before sleep, in which practice we find the most remote origin of our Compline.
Contemporary Orthodox Practice
Compline takes two distinct forms: Small Compline and Great Compline. The two versions are quite different in length.
At Compline (whether Small or Great), a Canon to the Theotokos in the Tone of the Week will normally be read (these Canons will be found in the Octoechos). Services to saints in the Menaion that, for one reason or another, cannot be celebrated on the day assigned to them, may be chanted on the nearest convenient day at Compline. In such cases, the Canon for the Saint would be read together with the Canon to the Theotokos, followed by the Stichera to the saint from Vespers. There are also particular days (such as certain Forefeasts, Afterfeasts, and days during the Pentecostarion) that have special Canons for Compline composed for them.
The Office always ends with a mutual asking of forgiveness. In some traditions, most notably among the Russians, Evening Prayers (i.e., Prayers Before Sleep) will be read near the end of Compline. It is an ancient custom, practiced to this day on the Holy Mountain and in other monasteries, for everyone present at the end of Compline to venerate the Relics and Icons in the church, and receive the priest's blessing.
Small Compline is served on most nights of the year (i.e., those nights on which Great Compline is not served). On the eves of Sundays and feasts with All-Night Vigil, Compline may be either read privately or suppressed altogether. Among the Greeks, who do not normally hold an All-Night Vigil on Saturday evenings, Compline is said as normal.
The service is composed of three Psalms (50, 69, 142), the Small Doxology, the Nicene Creed, the Canon followed by It is Truly Meet, the Trisagion, Troparia for the day, Lord, have mercy (40 times), the Prayer of the Hours, the Supplicatory Prayer of Paul the Monk, and the Prayer to Jesus Christ of Antiochus the Monk. Then the mutual forgiveness and final blessing by the Priest. After this, there is a Litany and the veneration of Icons and Relics.
Great Compline is a penitential office which is served on the following occasions:
- Tuesday and Thursday nights of Cheesefare Week (the week before Great Lent)
- Monday through Thursday nights of Great Lent 
- Friday nights of Great Lent 
- Monday and Tuesday of Holy Week
- Monday through Friday during the lesser Lenten seasons: Nativity Fast, Apostles' Fast and Dormition Fast 
- The Eves of certain Great Feasts, as a part of the All-Night Vigil:
Unlike Small Compline, Great Compline has portions of the service which are chanted by the Choir  and during Lent the Prayer of St. Ephraim is said with prostrations. During the First Week of Great Lent, the Great Canon of Saint Andrew of Crete is divided into four portions and read on Monday through Thursday nights.
In Greek Prayer Books, a modified form of the Midnight Office is used for morning prayers for laymen, while a modified form of Small Compline is used for evening prayers.
Great Compline is composed of three sections, each beginning with the call to prayer, "O come, let us worship...":
- Psalms 4, 6, and 12; Glory..., etc.; Psalms 24, 30, 90; then the hymn "God is With Us" and troparia, the Creed, the hymn "O Most holy Lady Theotokos", the Trisagion and Troparia of the Day, Lord, have mercy (40 times), "More honorable than the cherubim..." and the Prayer of St. Basil the Great.
- Psalms 50, 101, and the Prayer of Manassses; the Trisagion, and Troparia of Repentance, Lord, have mercy (40 times), "More honorable than the cherubim..." and the Prayer of St. Mardarius.
- Psalms 69, 142, and the Small Doxology; then the Canon followed by It is Truly Meet, the Trisagion, the hymn "O Lord of Hosts, be with us...", Kyrie eleison (40 times), the Prayer of the Hours, "More honorable than the cherubim....", the Prayer of St. Ephraim, Trisagion, the Supplicatory Prayer of Paul the Monk, and the Prayer to Jesus Christ of Antiochus the Monk. Then the mutual forgiveness. Instead of the normal final blessing by the Priest, all prostrate themseles while the priest reads a special prayer intercessory prayer. Then the Litany and the veneration of Icons and Relics.
- Certain canons will call for It is Truly Meet to be replaced by the Irmos of the Ninth Ode.
- Here follow the Evening Prayers in places where they are said at Compline.
- Except for Wednesday of the Fifth Week. The Great Canon of Saint Andrew of Crete will have been read the evening before, and so Small Compline is appointed for that Wednesday night.
- Among the Greeks, Small Compline is served on every Friday evening of Great Lent; the Russians, however, serve Great Compline on Fridays, with some modifications (see n. 7, below). On Friday night of the Fifth Week of Great Lent, the Akathist to the Theotokos is solemnly chanted, so Small Compline on that night will be either read privately or suppressed.
- In some places, Great Compline will only be served on the first night of each of the Lesser Fasts.
- Except on Friday night, when most of these parts are read. There are also fewer prostrations on Friday night.
- On Monday through Thursday of the First Week of Great Lent, we begin first with Psalm 69, followed by the appropriate section of the Great Canon (in which case, Psalm 69 will be omitted in the Third Part).
- Or, if it is the eve of a Great Feast, the Kontakion of the day.
- On Great Feasts, the order of Great Compline ends here, and we continue the All-Night Vigil with the Litia.