The Archbishop of Canterbury, Rowan Williams, sparked a stormy debate when he appeared to suggest that some aspects of Sharia law should be adopted in the UK.
Sharia law complements other legal system in several countries
Sharia is a legal and social code for Muslims to live by. It does not offer a fixed set of rules, and there are several differing interpretations. But it has proved controversial in the West for the extreme nature of some of its punishments.
A UK government minister called the idea that Sharia legal system could be introduced alongside the current English system "unacceptable".
In fact, parallel systems do exist in several countries. In some, Sharia exists as an alternative system, and in others it has been incorporated into the existing legal code. We examined different cases to see how it can work in practice.
They are Nigeria, Aceh in Indonesia, India and Egypt.
The legal structure in Nigeria has its roots in colonial times, when British rulers introduced an Islamic code to co-exist alongside the colonial, English-based judicial system and customary law (supposed to apply to non-Muslim ethnic minority groups).
In 2000, 12 predominantly Muslim northern states adopted additional, stricter Sharia punishments into their penal code - including amputation for theft and, in theory, death for adultery or sodomy.
In part, say correspondents, this was an expression of tensions with the Christian leadership of the country in the south. But it was also a popular - and populist - move that reflected disgust with rampant corruption and misrule.
The Sharia courts mostly deal with domestic issues such as marriage and divorce. The Sharia code runs alongside the secular state system and - in theory at least, and usually in practice - citizens can choose which system they deal with.
The adoption of Sharia penalties in 2000 sparked riots among Christians in the north in which hundreds were killed.
There have been complaints that the Sharia police, the hisbah, have arbitrarily shut down bars serving alcohol, prevented lone women taking taxis with male drivers, or censored books or plays they deem critical of Islam or immoral.
Correspondents say there are also concerns that Sharia penalties infringe upon the constitutional rights enjoyed by Nigerians.
Flogging is a common form of punishment associated with Sharia law, and there have been amputations, though they are rare.
However, the harshest Sharia penalty - death by stoning - has never been carried out. And for many Muslims in Nigeria, the Sharia courts may provide a quicker, more efficient route to justice than the alternatives.
Partial Sharia law was introduced in the Indonesian province of Aceh as part of an autonomy deal offered by the Jakarta government following a decades-long separatist campaign by rebels. It has been enforced rigorously since the 2004 tsunami.
Province on the north-western tip of Sumatra
High percentage of Muslims, and only province where Sharia law allowed
Separatist rebels fought decades-long campaign against Jakarta
December 2004 Indian Ocean tsunami devastated region
New peace talks led to August 2005 agreement
The province has a higher proportion of Muslims than other areas of Indonesia, and many Acehnese practise a stricter version of Islam.
The people of Aceh are banned from gambling, drinking alcohol, and pre-marital sex, and both men and women are expected to dress modestly, with women required to wear headscarves in public.
Offenders may face corporal punishment. There have been calls for the introduction of amputation and stoning for serious crimes, though this does not appear imminent.
The introduction of Sharia law saw the creation of a Sharia police force which complements the regular federal police force in Aceh. But the Sharia force's powers were curtailed after accusations they were enforcing the law overzealously.
It is unclear how much support the imposition of Islamic law has in the province.
Some analysts suggest that in a society long characterised by violence and lawlessness, it was popular at first, but that many Acehnese have been turned off by public whippings and public humiliation, and the strict imposition of headscarves on women. There seems little appetite in the rest of Indonesia for any similar initiative.
Again, there is debate about whether the Sharia rules infringe people's constitutional rights - particularly those of women and the poor.
During British colonial rule, India came under a common criminal code of laws, drafted in 1830 by historian Thomas Babington Macaulay.
But the rulers did not attempt to replace the personal laws of various sects and religions with a uniform civil code or common laws. This included Muslim personal laws.
RELIGION IN INDIA
India has 140m Muslims, the third largest Muslim population in the world after Indonesia and Pakistan
They comprise 13% of the population
Christians comprise 2.3% of the population
India does not have an official state religion; it enshrines the right to practise, preach, and propagate any religion
After India became independent, the constitution said that the state shall "endeavour to secure for its citizens a uniform civil code throughout the territory of India".
But India's first Prime Minister, Jawaharlal Nehru, allowed each religious community to retain its own civil laws governing marriages, divorces, births, deaths and inheritances.
This was done to maintain religious harmony after the bloodletting between the Hindu and Muslim communities during the Partition.
There has been no change in the Muslim laws.
In 1973, the All India Muslim Personal Law Board was constituted for protection of these laws. The organisation has split with Shias and some women breaking away to form their own personal law boards.
A growing number of community members are seeking to reform Muslim personal laws, chief among them the custom of triple talaq, which permits Muslim husbands to divorce by saying "I divorce you" three times.
At the same time, not all Muslim personal laws have been followed strictly by the community: only 2% of Muslim men in India, for example have more than one wife, though the community's personal law allows men to take more than one spouse.
It is not easy for the government to change community laws - the constitution says that the state cannot alter any community laws without the backing of 75% of the its members.
Sharia law in Egypt applies only in personal status issues - such as marriage, divorce, inheritance, and custody of children. Otherwise the legal system is entirely a secular one, on the model of the French legal system.
Egypt incorporates Islamic law into its constitution by making Islam the official religion of the country and Islamic jurisprudence the principal source of legislation.
Sharia courts and judges, qadis, are run and licensed by the Egyptian Ministry of Justice, not by mosques.
Islamists in Egypt are pressing for Sharia law to be applied in all areas of the legal system.
A non-religious Supreme Court operates above the Sharia personal status courts and the secular criminal courts.
Religious minorities in Egypt are governed under separate personal status laws and courts.
Coptic Christians in Egypt marry under Christian law, and foreigners marry under the laws of their countries of origin.