It was the 21st such gathering of Catholic leaders in the history of the church.
It opened 50 years ago today in Rome's St Peter's Basilica.
The effect of Vatican II on the world's 1.96 billion Catholics has been tumultuous, writes Paul Collins in The Brisbane Times.
It was a genuinely worldwide gathering of 2500-2800 bishops from almost every country. It met over four sessions between 1962 and 1965.
It had been called by Pope John XXIII who realised Catholicism had become stultified, turned in on itself and cut off from the modern world. He wanted ''to throw open the windows'' and was critical of ''the prophets of doom, who are always forecasting disaster''.
He wanted the church to reach out ''fearlessly'' to the world. The Vatican II theologian Yves Congar said the church was being challenged ''by the world to rejoin it in order to speak validly of Jesus Christ''.
For four decades before 1962, Catholic scholars had been evolving a renewed approach to theology, the Bible, worship, religious formation, spirituality and faith which during Vatican II gradually influenced the views of the majority of the bishops.
They developed a vision of the church open to the contemporary world, ministering to its needs, speaking of faith and spirituality in terms that made sense to people, and emphasising the priority of peace, social justice and equity.
The real struggle was to ensure that Vatican II documents reflected these priorities. But this was not going to be easy.
The ''prophets of doom'' were entrenched at the highest levels of the church and they resisted this more open vision to the end.
Pope John died in June 1963. He was succeeded by Paul VI (1963-1978) who continued the council and generally supported the thrust of the large majority of the bishops.
But psychologically Paul was a hesitant man, a ditherer even. He was afraid of alienating reactionary bishops especially those from the Roman Curia, the Vatican bureaucracy. He feared that they would walk out of the council, leading to schism.
The result was that key documents of Vatican II were compromises. For instance, the document on the nature of the church first envisions the church as a community drawn together by God's spirit and built not on the hierarchy but the people. But then, almost as if the community model didn't exist, the church is characterised as an authority-driven, clerical institution dominated by Pope, bishops and the Vatican.
These models are not only mutually exclusive, they are corrosive.
The same kind of disjunction can be found in other documents, particularly the one on the priesthood.