Pope Benedict has led the Catholic Church since 2005, and his papacy has reflected his belief that the Catholic Church should retain its core traditional, conservative values in an era of rapid change.
He rejected calls for a debate on the issue of clerical celibacy, and reaffirmed the ban on Communion for divorced Catholics who remarry. He has also said the Church's strict positions on abortion, euthanasia and gay partnerships were "not negotiable".
This outspoken orthodoxy has divided liberals and more traditional Catholics, while the recent leaking of personal documents suggests a lack of control over the machinations of the Vatican.
How has Benedict XVI managed the world's largest Christian community? We asked six scholars and analysts for their perspective on key areas of the pontificate.
Governance - Clifford Longley
Pope Benedict was clearly shocked and disappointed that one of those closest to him, his butler Paolo Gabriele, had been a source of the leaked Vatican documents that revealed a state of turmoil inside the central government of the Catholic Church.
The Curia - the central government of the Church - appeared to be riddled with rival factions and there were accusations of corruption in high places.
Observers had long labelled the Vatican a dysfunctional institution - now the details were laid bare.
Many curial officials, men dedicated to the good of the Church, felt frustrated and exasperated by the institutional sclerosis they encountered.
Pope Benedict may well have realised that in his 86th year he no longer had the energy to do what needed to be done to sort this all out.
That, and his inability to go on travelling the great distances that the job demands, may well have been what persuaded him to abdicate - for the good of the Church.
The reform of the Vatican, which he had only begun at the margins, has a long way to go yet.
Decentralisation is now imperative. His successor has a huge and unenviable task.
Evangelisation - Austen Ivereigh
In speeches, addresses and teaching documents Benedict XVI considered how to present the Gospel to cultures steeped in Christian values yet often hostile to the Church.
This specific challenge, known as the "new evangelisation" ("new" referring to the context rather than the message), was one of Pope Benedict's priorities: he created a new Vatican department to promote it, and last October called together the world's bishops to agree how to take it forward.
The cardinals gathered in Rome in March will regard the new evangelisation as one of the priorities for the next pope.
They have no choice. Equality laws such as same-sex marriage make Christians and church organisations vulnerable to lawsuits and anti-discrimination claims, and threaten to marginalise further the presence of the Church in public life.
It is highly likely, in fact, that Benedict XVI's successor will be one of those listed as members of the Department for the Promotion of the New Evangelisation.
Women - Gemma Simmonds
There have been marked successes in Pope Benedict's pontificate, but advancing the role of women in the Catholic Church has not been one of them.
The most glaring failure in this respect has been the Apostolic Visitation of the nuns of North America. The US owes a huge debt to generous, heroic sisters who have dedicated their lives to offering education, healthcare and pastoral provision only to be subjected to an intrusive, inherently hostile process of investigation for alleged doctrinal errors. The contrast between their treatment and that of paedophile clergy has caused widespread scandal.
Some remarkable women have risen to posts in the Vatican, but others considered "difficult" have been removed. Pope Benedict declared venerable Englishwoman Mary Ward (1585-1645) who wrote, "There is no such difference between men and women that women may not do great things." There has been little room for them to try in this pontificate.
In the post-mortems being carried out on his term of office, the media focus has inevitably been on Pope Benedict's handling of contentious issues such as gay marriage and adoption, homosexuality and other social issues. He is bound to appear in a poor light to those unwilling to consider him in context.
The Catholic Church is never going to view marriage except from the perspective of a sacrament, an unbreakable bond between one man and one woman, open to the God-given gift of children. In so far as this was the understanding of Jesus Christ, it will be that of his vicar on Earth. But Scripture also teaches that "God is love and all who live in love live in God, and God lives in them".
The experience of many Catholics is that this love in which God dwells can be found in multiple forms, all working towards the faithfulness and self-giving commitment to which the sacrament of marriage points. What was needed was a brilliant theological mind able to reconcile both these truths and make sense of them, but this did not emerge clearly from Pope Benedict's writings.
What did emerge repeatedly, and was shamefully under-reported, was his incisive critique of global capitalism and its toxic social effects, his unflinching opposition to war and current world conflicts, especially the war in Iraq, and his forceful articulation of the ecological imperative. In its way his writing on these issues is every bit as radical as his resignation, if only the world could be bothered to look.
Foreign and interfaith relations - James Roberts
The Pope sparked outrage in some Muslim circles in 2006 when, in a lecture discussing faith, reason and violence, he quoted from a 14th-Century Byzantine emperor who labelled Muhammad "evil and inhuman".
However, after a time, the successful Muslim-Christian dialogue, "A Common Word" was established, and is ongoing.
In more recent years, the Pope has placed much emphasis on advocacy of religious freedom - notably in the face of horrific persecution of Christians in Pakistan and Nigeria, and the refusal by Saudi Arabia to allow its Christian "guest workers" any religious rights at all.
He has pleaded with the Middle Eastern powers to grant Christians the freedoms that will allow them to stay, but the exodus from that region, and the persecution, continues.
Little noted among the successes of the Benedict XVI pontificate has been the improvement in Catholic-Jewish relations. His papacy has been characterised by the "best relations ever between Church and Rabbinate", according to Israel's Chief Rabbi Yona Metzger.
Israel's Chief Sephardic Rabbi Shlomo Amar called the Pope a "justice warrior" and said he should be commended for his "steadfast fight against any expression of anti-Semitism and Holocaust denial, inside and outside the Church."
Sex abuse - David Clohessy
We were never pleased with how Pope Benedict handled the clergy sex abuse and covered up the crisis. That is why we filed suit against him and other Vatican officials in the International Criminal Court.
Since 1981, when he headed the Congregation for the Doctrine of the Faith, Benedict had primary responsibility for dealing with the clergy sex crimes.
His refusal to decisively address the epidemic - and discipline Church officials who protected predator priests - was exacerbated when he became Pope.
The next pontiff must do more to safeguard children.
He should stop issuing apologies and making gestures, and instead demote bishops who continue to conceal heinous crimes.
And he should insist that prelates work with secular authorities to craft and pass stronger child sex laws across the globe.
Theology - Fr Stephen Wang
The key to Pope Benedict's theology is the idea of "connection" or "continuity".
How do you preserve the fundamental connections between faith and reason, between the past and the present, between the human and the divine? How do you avoid a rupture that would betray the Christian vision and impoverish everyday life?
His first encyclical letter surprised everyone by being a meditation on love. The joy of human love ("eros" or erotic love) leads us to a deeper, sacrificial love ("agape"), that finds its true fulfilment in the love of Jesus Christ on the Cross.
The human and the divine connect; they are not in opposition.
The worship of the Church, whatever new forms it takes, needs to connect with its 2,000-year history.
The moral values of the Church, even if they are expressed in new ways, need to be rooted in the wisdom of the Bible and the Christian tradition.
And Catholic teaching, which is always developing, should never betray the sure faith that has been handed down through the centuries. He believed in renewal and reform, but always in continuity with the past.
He called on Catholics to deepen their faith, through studying the Catechism. He encouraged the secularised West not to become trapped in a dictatorship of relativism - where everything is allowed but nothing has any meaning.
For Pope Benedict, Christianity is a revealed religion, not something we create for ourselves. It surprises and startles us.
No wonder that his last published work was about discovering the face of God in Jesus Christ, the child of Bethlehem.