The seven Ecumenical Councils recognized by the Orthodox Church are:
Nicea I (AD. 325) This Council was called to deal with the heresy of Arianism - the teaching that the Word and Son of God is a created being. The first part of the Nicene Creed was drafted here. The hero of Nicea I was St. Athanasius of Alexandria, whose theology was decisive even though, as a deacon at the time, he could not vote.
Constantinople I (AD. 381) This Council expanded and completed the Nicene Creed and affirmed the divinity of the Holy Spirit. The theology of the Cappadocian Fathers - St. Basil the Great, St. Gregory the Theologian, and St. Gregory of Nyssa - was particularly influential here.
Ephesus (AD. 431) This Council condemned the teachings of Patriarch Nestorius of Constantinople, who refused to accept the unity of humanity and divinity in the person of Christ and who refused to call the Virgin Mary Theotokos. St. Cyril, Archbishop of Alexandria, was the hero of this Council.
Chalcedon (AD. 451) This Council was called to combat the opposite heresy of Nestorianism: Monophysitism. According to the Monophysites, Christ's divine nature swallowed up His human nature, leaving Him with only one nature. The bishops accepted the Tome of Pope St. Leo the Great along with the theology of St. Cyril of Alexandria as the standard of Orthodox thought concerning the person of Christ. The Council decreed that in Christ the divine and human natures exist without "mixture, confusion, division, or separation."
Constantinople II (AD. 553) This Council further elaborated on the decisions of the Council of Chalcedon. In addition, some of the teachings of Origen of Alexandria, such as the pre-existence of souls, were condemned.
Constantinople III (AD. 681) This Council condemned the heresy of Monothelitism, which held that Christ had only one will. The bishops affirmed that Christ has a perfect human will as well as a perfect divine will, thus affirming His full humanity. Pope Honorius of Rome was condemned as a heretic for his support of the Monothelites.
Quinisext (AD. 692) This Council, also called the Council in Trullo, is considered a continuation of the Fifth and Sixth Councils and not a separate Council unto itself. Among other things, it reaffirmed the condemnation of the teachings of Origen.
Nicea II (A.D.787) This Council was called to decide the appropriateness of using icons in the Church. The bishops decreed that the veneration (not worship) of icons was necessary to preserve a proper understanding of the Incarnation.
Source: The Faith …… Author Clark Carlton