I. FIRST COUNCIL OF NICAEA
Year: 325
Summary: The Council of Nicaea lasted two months and twelve days. Three hundred and eighteen bishops were present. Hosius, Bishop of Cordova, assisted as legate of Pope Sylvester. The Emperor Constantine was also present. To this council we owe the Nicene Creed, defining against Arius the true Divinity of the Son of God (homoousios), and the fixing of the date for keeping Easter (against the Quartodecimans).

II. FIRST COUNCIL OF CONSTANTINOPLE
Year: 381
Summary: The First General Council of Constantinople, under Pope Damasus and the Emperor Theodosius I, was attended by 150 bishops. It was directed against the followers of Macedonius, who impugned the Divinity of the Holy Ghost. To the above-mentioned Nicene Creed it added the clauses referring to the Holy Ghost (qui simul adoratur) and all that follows to the end.

III. COUNCIL OF EPHESUS
Year: 431
Summary: The Council of Ephesus, of more than 200 bishops, presided over by St. Cyril of Alexandria representing Pope Celestine I, 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
Summary: The Council of Chalcedon -- 150 bishops under Pope Leo the Great and the Emperor Marcian -- defined the two natures (Divine and human) in Christ against Eutyches, who was excommunicated.

V. SECOND COUNCIL OF CONSTANTINOPLE
Year: 553
Summary: The Second General Council of Constantinople, of 165 bishops under Pope Vigilius and Emperor Justinian I, condemned the errors of Origen and certain writings (The Three Chapters) of Theodoret, of Theodore, Bishop of Mopsuestia and of Ibas, Bishop of Edessa; it further confirmed the first four general councils, especially that of Chalcedon whose authority was contested by some heretics.

VI. THIRD COUNCIL OF CONSTANTINOPLE
Years: 680-681
Summary: The Third General Council of Constantinople, under Pope Agatho and the Emperor Constantine Pogonatus, was attended by the Patriarchs of Constantinople and of Antioch, 174 bishops, and the emperor. 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
Summary: The Second Council of Nicaea 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. Between 300 and 367 bishops assisted.

VIII. FOURTH COUNCIL OF CONSTANTINOPLE
Year: 869
Summary: The Fourth General Council of Constantinople, under Pope Adrian II and Emperor Basil numbering 102 bishops, 3 papal legates, and 4 patriarchs, 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; it condemned Photius who had unlawfully seized the patriarchal dignity. 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
Summary: The First Lateran Council, the first 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
Summary: The Second Lateran Council 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
Summary: The Third Lateran Council took place under Pope Alexander III, Frederick I being emperor. There were 302 bishops present. It condemned the Albigenses and Waldenses and issued numerous decrees for the reformation of morals.

XII. FOURTH LATERAN COUNCIL
Year: 1215
Summary: The Fourth Lateran 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 (symbol) against the Albigenses (Firmiter credimus), 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
Summary: The First General Council of Lyons was presided over by Innocent IV; the Patriarchs of Constantinople, Antioch, and Aquileia (Venice), 140 bishops, Baldwin II, Emperor of the East, and St. Louis, King of France, assisted. 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
Summary: The Second General Council of Lyons was held by Pope Gregory X, the Patriarchs of Antioch and Constantinople, 15 cardinals, 500 bishops, and more than 1000 other dignitaries. It effected a temporary reunion of the Greek Church with Rome. The word filioque was added to the symbol 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
Summary: The Council of Vienne was held in that town 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 authorities), and 3 kings -- Philip IV of France, Edward II of England, and James II of Aragon -- were present. The synod 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
The Council of Constance was held during the great Schism of the West, with the object of ending the divisions in the Church. It became legitimate only when Gregory XI 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. The rightful pope confirmed the former decrees of the synod against Wyclif and Hus. This council is thus ecumenical only in its last sessions (42-45 inclusive) and with respect to the decrees of earlier sessions approved by Martin V.

XVII. COUNCIL OF BASLE/FERRARA/FLORENCE
Years: 1431-1439
Summary: The Council of Basle met first in that town, Eugene IV being pope, and Sigismund Emperor of the Holy Roman Empire. Its object was the religious pacification of Bohemia. Quarrels with the pope having arisen, the council was transferred first to Ferrara (1438), then to Florence (1439), where a short-lived union with the Greek Church was effected, the Greeks accepting the council's definition of controverted points. 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
Summary: The Fifth Lateran Council sat from 1512 to 1517 under Popes Julius II and Leo X, the emperor being Maximilian I. Fifteen cardinals and about eighty archbishops and bishops took part in it. Its decrees are chiefly disciplinary. A new crusade against the Turks was also planned, but came to naught, owing to the religious upheaval in Germany caused by Luther.

XIX. COUNCIL OF TRENT
Years: 1545-1563
Summary: The Council of Trent lasted eighteen years (1545-1563) under five 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
Summary: The Vatican Council was summoned by Pius IX. It met 8 December, 1869, and lasted till 18 July, 1870, when it was adjourned; it is still (1908) unfinished. There were present 6 archbishop-princes, 49 cardinals, 11 patriarchs, 680 archbishops and bishops, 28 abbots, 29 generals of orders, in all 803. Besides important canons relating to the Faith and the constitution of the Church, the council decreed the infallibility of the pope when speaking 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.

XXI. SECOND VATICAN COUNCIL
Years: 1962-1965

Source: The Catholic Encyclopedia