The Great Sanhedrin (which in Greek means 'a sitting together' or 'council') was the supreme council of the Jews who met within Jerusalem's temple in a place known as the chamber of hewn stones (although they sometimes met in the house of the High Priest). They were like the U.S. Supreme court in that it was the final authority on decisions that affected the religious and political life of all Jews. The council convened each day of the week except the Sabbath and the annual Biblical Holy Days.
In New Testament times, the Sanhedrin was composed of seventy-one members: the High Priest, a vice chief justice and sixty-nine general members. Only chief priests, elders, and scribes were eligible to sit on the council. Members were required to possess scholarship, modesty, strength, courage, and popularity among their fellow men. Their rulings were binding (for a time) on all Jews scattered throughout the world. Their jurisdiction, however, was greatly curtailed by Herod the Great and then by the Roman Empire.
The Sanhedrin is traditionally viewed as the last institution that commanded universal Jewish authority among the Jewish people. Until the time when Judea was subjected to the Romans, it had the power of life and death. In New Testament times, although it retained the power of passing sentence, the Roman Empire retained the right to accept or reject their verdicts and to ultimately decide and carry out any penalties. Since their dissolution in 358 A.D. by imperial decree, there have been several attempts to re-establish it either as a self-governing body or as a puppet of a sovereign government.
In the New Testament, they are variously referred to as "the chief priests, the elders, and all the council" (Matthew 26:59, NKJV throughout) and the "chief priests, the scribes, and the elders of the people" (Matthew 26:3). They are also called the "chief priests and elders of the people" (Matthew 27:1) and in several places referred to as "the council" (Mark 15:1, Matthew 5:22, Acts 5:21, etc.).
This group was THE driving force that insured Jesus was betrayed, falsely arrested, falsely accused, unjustly judged, and condemned to a death on the cross. It was members of the council who bribed Judas Iscariot with thirty pieces of silver to betray Jesus (Matthew 26:14 - 16). It was they who, contrary to Jewish law, had Jesus arrested without a formal accusation of wrong from at least two people (Deuteronomy 19:15).
During Jesus' first trial the charge of blasphemy against God was used by the Sanhedrin council to justify giving him a death sentence (Mark 14:61-64, Matthew 26:62-66). Yet, when Jesus was brought to the Romans for punishment, they CHANGED the charge against him to treason (Luke 23:1 - 3, John 19:12)! It was members of the council who, when Pontius Pilate asked a crowd whether Jesus or Barabbas should be set free, got the crowd to select Barabbas (Matthew 27:15 - 23).