So far, the Catholic Church has had 21 Ecumenical Councils. Somewhere in our collective memory Catholics have only heard of one council; so it only makes sense to learn a little about all these other councils that have guided the Church and defined her dogmas.
The 21 Ecumenical Councils
I. FIRST COUNCIL OF NICAEA
Year: 325
Overview: This Council lasted two months and twelve days. Three hundred and eighteen bishops attended it. Hosius, Bishop of Cordova, assisted as legate of Pope Sylvester. The Emperor Constantine was present at the council. To this council we owe the Nicene Creed, which defined the Divinity of the Son of God against the heresy of Arius (Arianism).
II. FIRST COUNCIL OF CONSTANTINOPLE
Year: 381
Overview: Held under Pope Damasus and the Emperor Theodosius I; it was attended by 150 bishops. It was in response to the followers of Macedonius, who impugned the Divinity of the Holy Ghost. It added to the Nicene Creed the clauses referring to the Holy Ghost (qui simul adoratur) and all that follows to the end.
III. COUNCIL OF EPHESUS
Year: 431
Overview: Presided over by St. Cyril of Alexandria representing Pope Celestine I, it was attended by more than 200 bishops. This council defined the true personal unity of Christ, declared Mary the Mother of God (theotokos) against Nestorius, Bishop of Constantinople, and renewed the condemnation of Pelagius.
IV. COUNCIL OF CHALCEDON
Year: 451
Overview: Under Pope Leo the Great and the Emperor Marcian , 150 bishops defined the two natures (Divine and human) in Christ against Eutyches, who was excommunicated.
V. SECOND COUNCIL OF CONSTANTINOPLE
Year: 553
Overview: Led by Pope Vigilius and Emperor Justinian I, attended by 165 bishops this council condemned the errors of Origen and certain writings, it further confirmed the first four general councils, especially that of Chalcedon whose authority was contested by those outside the Church.
VI. THIRD COUNCIL OF CONSTANTINOPLE
Years: 680-681
Overview: Under Pope Agatho and the Emperor Constantine Pogonatus, this council was attended by the Patriarchs of Constantinople and of Antioch, and 174 bishops. It put an end to Monothelitism by defining two wills in Christ, the Divine and the human, as two distinct principles of operation. It anathematized Sergius, Pyrrhus, Paul, Macarius, and all their followers.
VII. SECOND COUNCIL OF NICAEA
Year: 787
Overview: This council was convoked by Emperor Constantine VI and his mother Irene, under Pope Adrian I, and was presided over by the legates of Pope Adrian; it regulated the veneration of holy images. It was assisted by approximately 360 bishops.
VIII. FOURTH COUNCIL OF CONSTANTINOPLE
Year: 869
Overview: Under Pope Adrian II and Emperor Basil, this council was assisted by 102 bishops, 3 papal legates, and 4 patriarchs. They consigned to the flames the Acts of an irregular council (conciliabulum) brought together by Photius against Pope Nicholas and Ignatius the legitimate Patriarch of Constantinople. The Photian Schism, however, triumphed in the Greek Church, and no other general council took place in the East.
IX. FIRST LATERAN COUNCIL
Year: 1123
Overview: This was the first council held at Rome, met under Pope Callistus II. About 900 bishops and abbots assisted. It abolished the right claimed by lay princes, of investiture with ring and crosier to ecclesiastical benefices and dealt with church discipline and the recovery of the Holy Land from the infidels.
X. SECOND LATERAN COUNCIL
Year: 1139
Overview: This council too was held at Rome under Pope Innocent II, with an attendance of about 1000 prelates and the Emperor Conrad. Its object was to put an end to the errors of Arnold of Brescia.
XI. THIRD LATERAN COUNCIL
Year: 1179
Overview: Under Pope Alexander III and the Emperor Frederick I, it was assisted by 302 bishops. It condemned the Albigenses and Waldenses and issued numerous decrees for the reformation of morals.
XII. FOURTH LATERAN COUNCIL
Year: 1215
Overview: This council was held under Innocent III. There were present the Patriarchs of Constantinople and Jerusalem, 71 archbishops, 412 bishops, and 800 abbots, the Primate of the Maronites, and St. Dominic. It issued an enlarged creed against the Albigenses, condemned the Trinitarian errors of Abbot Joachim, and published 70 important reformatory decrees. This is the most important council of the Middle Ages, and it marks the culminating point of ecclesiastical life and papal power.
XIII. FIRST COUNCIL OF LYONS
Year: 1245
Overview: Presided over by Innocent IV; the Patriarchs of Constantinople, Antioch, and Aquileia (Venice), it was attended by 140 bishops, Baldwin II, Emperor of the East, and St. Louis, King of France. It excommunicated and deposed Emperor Frederick II and directed a new crusade, under the command of St. Louis, against the Saracens and Mongols.
XIV. SECOND COUNCIL OF LYONS
Year: 1274
Overview: Held by Pope Gregory X, the Patriarchs of Antioch and Constantinople, 15 cardinals, 500 bishops, and more than 1000 other dignitaries attended this council. It effected a temporary reunion of the Greek Church with Rome. The word filioque was added to the creed of Constantinople and means were sought for recovering Palestine from the Turks. It also laid down the rules for papal elections.
XV. COUNCIL OF VIENNE
Years: 1311-1313
Overview: Held in Vienne in France by order of Clement V, the first of the Avignon popes. The Patriarchs of Antioch and Alexandria, 300 bishops (114 according to some sources), and 3 kings -- Philip IV of France, Edward II of England, and James II of Aragon -- were present. The council dealt with the crimes and errors imputed to the Knights Templars, the Fraticelli, the Beghards, and the Beguines, with projects of a new crusade, the reformation of the clergy, and the teaching of Oriental languages in the universities.
XVI. COUNCIL OF CONSTANCE
Years: 1414-1418
Overview: Held during the great Schism of the West, with the object of ending the divisions in the Church. This council became legitimate when Gregory XII had formally convoked it. Owing to this circumstance it succeeded in putting an end to the schism by the election of Pope Martin V, which the Council of Pisa (1403) had failed to accomplish on account of its illegality.
XVII. COUNCIL OF BASLE/FERRARA/FLORENCE
Years: 1431-1439
Overview: Eugene IV being pope, and Sigismund Emperor of the Holy Roman Empire convened this council with the object of religious pacification of Bohemia. Quarrels with the pope having arisen, the council was transferred first to Ferrara (1438), then to Florence (1439).. The Council of Basle is only ecumenical till the end of the twenty-fifth session, and of its decrees Eugene IV approved only such as dealt with the extirpation of heresy, the peace of Christendom, and the reform of the Church, and which at the same time did not derogate from the rights of the Holy See.
XVIII. FIFTH LATERAN COUNCIL
Years: 1512-1517
Overview: The council lasted from 1512 to 1517 under Popes Julius II and Leo X, and the Emperor Maximilian I . Fifteen cardinals and about eighty archbishops and bishops took part in it. Its decrees were chiefly disciplinary. A new crusade against the Turks was also planned, but it didn’t take off, owing to the religious upheaval in Germany caused by Luther.
XIX. COUNCIL OF TRENT
Years: 1545-1563
Overview: This council lasted eighteen years from 1545 to 1563. It was led by five different popes: Paul III, Julius III, Marcellus II, Paul IV, and Pius IV, and under the Emperors Charles V and Ferdinand. There were present 5 cardinal legates of the Holy See, 3 patriarchs, 33 archbishops, 235 bishops, 7 abbots, 7 generals of monastic orders, and 160 doctors of divinity. It was convoked to examine and condemn the errors promulgated by Luther and other Reformers, and to reform the discipline of the Church. Of all councils it lasted longest, issued the largest number of dogmatic and reformatory decrees, and produced the most beneficial results.
XX. FIRST VATICAN COUNCIL
Years: 1869-1870
Overview: Vatican I was convened under Pius IX. It met 8 December, 1869, and lasted till 18 July, 1870, when it was adjourned; it remained unfinished. There were present 6 archbishop-princes, 49 cardinals, 11 patriarchs, 680 archbishops and bishops, 28 abbots, 29 generals of orders, in all 803. This council decreed the infallibility of the Holy Father when he speaks ex cathedra, i.e. when as shepherd and teacher of all Christians, he defines a doctrine concerning faith or morals to be held by the whole Church.
Source: The Catholic Encyclopedia