A memorial service (Greek: μνημόσυνον, mnemósynon, "memorial"; Slavonic: панихида, panikhída, from Greek παννυχίς, pannychis, "vigil" (etymologically "all-nighter"); Romanian: parastas and Serbian парастос, parastos, from Greek παράστασις, parástasis) is a liturgical solemn service for the repose of the departed in the Eastern Orthodox and Byzantine Catholic churches.

In the Eastern Church, the various prayers for the departed have as their purpose praying for the repose of the departed, comforting the living, and reminding the living of their own mortality and the brevity of this earthly life. For this reason, memorial services have an air of penitence about them. They tend to be served more frequently during the four fasting seasons.

If the service is for an individual, it is often held at the deceased's graveside. If it is a general commemoration of all the departed, or if the individual's grave is not close by, the service is held in a church, in front of a special small, free-standing "memorial table", to which is attached an upright crucifix and with a candelabra for the faithful to put lighted candles.

The deacon (or, if there is no deacon, the priest) swings the censer throughout almost the entire service, while all stand holding lighted candles. Near the end of the service, during the final troparia, all either extinguish their candles, or place them in a candle holder by the memorial table. Each candle symbolizes the individual soul, which, as it were, each person holds in his own hand. The extinguishing (or giving up) of the candle, at the end of the service, symbolizes the fact that each person will have to surrender his soul, at the end of his life.

The service is composed of Psalms, ektenias (litanies), hymns and prayers. In its outline it follows the general order of Matins and is, in effect, a truncated funeral service. Some of the most notable portions of the service are the Kontakion of the Departed and the final singing of "Memory Eternal" (Slavonic: Vyechnaya Pamyat).

The memorial service is most frequently served at the end of the Divine Liturgy; however, it may also be served after Vespers, Matins, or as a separate service by itself. If the service is held separately, there are readings from the Pauline epistles and the Gospels, which are assigned by the day of the week; no readings, however, are assigned to Sunday because Sunday should emphasize the resurrection of Christ rather than the departed.

For the memorial service, koliva (a ritual food of boiled wheat) is often prepared and is placed in front of the memorial table or an icon of Christ. Afterwards, it is blessed by the priest, who sprinkles it with holy water. The koliva is then taken to the refectory and is served to all those who attended the service.

After an Orthodox Christian dies there are special "Prayers for the Departure of the Soul" that are said by the priest. Then the family or friends of the departed will wash and dress the body and it is placed in the casket after which a special expanded memorial service called the First Panikhida is celebrated, following which the reading of the Psalter commences and continues uninterrupted until the funeral.