The Great Fast or Lent is the period of preparation leading up to Holy Week and Pascha. The Lenten Triodion governs the divine services of Great Lent as well as those of the Weeks of Preparation preceding Great Lent. Lent is a Middle English word meaning "spring." The Great Fast has come to be called Lent by association; it is called "great" to distinguish it from the other fasts.

Observance of Great Lent is characterized by abstention from many foods, intensified private and public prayer, personal improvement, and almsgiving. The foods traditionally abstained from are meat and dairy products, fish, wine and oil. (According to some traditions, only olive oil is abstained from; in others, all vegetable oils.) Since strict fasting is canonically forbidden on the Sabbath and the Lord's Day, wine and oil are permitted on Saturdays and Sundays. If the Feast of the Annunciation falls during Great Lent, then fish, wine and oil are permitted on that day.

Besides the additional liturgical celebrations described below, Orthodox Christians are expected to pay closer attention to their private prayers and to say more of them more often. The Fathers have referred to fasting without prayer as "the fast of the demons" since the demons do not eat according to their incorporeal nature, but neither do they pray.

During the weekdays of Great Lent, there is a liturgical fast when the eucharistic Divine Liturgy is not celebrated. However, since it is considered especially important to receive the Holy Mysteries during this season the Liturgy of the Presanctified Gifts, also called the Liturgy of St. Gregory the Dialogist, may be celebrated on Wednesdays and Fridays. At this vesperal service some of the Body and Blood of Christ reserved the previous Sunday is distributed. On Saturday and Sunday the Divine Liturgy may be celebrated as usual, although on Sundays the more solemn Liturgy of St. Basil the Great is used in place of that of St. John Chrysostom.

Like the observation of Lent in the West, Great Lent itself lasts for forty days, but unlike the West, Sundays are included in the count. It officially begins on Monday seven weeks before Pascha and concludes on the eve of Lazarus Saturday, the day before Palm Sunday. However, fasting continues for the following week, known as Passion Week, Great Week or Holy Week, up until Pascha.

The original purpose of the pre-Pascha fast (now known as Great Lent) was the fasting of catechumens who were being prepared for baptism and entry into the Church. However, it quickly became a time for those who were already Christian to prepare for the feast of the Resurrection of Christ. It is the living symbol of man's entire life which is to be fulfilled in his own resurrection from the dead with Christ. It is a time of renewed devotion: of prayer, fasting, and almsgiving. It is a time of repentance, a real renewal of minds, hearts and deeds in conformity with Christ and his teachings. It is the time, most of all, of return to the great commandments of loving God and neighbors.