From LawGuru Wiki
 United States
Generally, under the laws of the various United States, a person can adopt any name desired for any reason as long as the name is not slanderous or otherwise not prohibited by the principle of freedom of speech, and as long as the name change is not for the purpose of committing a crime such as for fraudulently evading paying one's bills. There are differences in specific requirements from state to state. If someone wishes to obtain a formal order changing their name (which would be applied for in a state court), it is necessary to plead that the name change is not for a fraudulent or other illegal purpose. The applicant may be required to give a more or less reasonable explanation for wanting to change their name. Generally the court (that is, the judge) has full judicial discretion to grant or deny a change of name, especially if the name change is for frivolous or immoral purposes, such as changing one's name to "God," "Penis," or "Copyright."
Under the federal immigration and nationality law, when immigrants apply for citizenship they are allowed to ask for their name to be changed upon the grant of citizenship. This allows them the opportunity to adopt a more native-sounding name. Likewise in some states upon divorce individuals are often allowed to return to the use of any prior surname (i.e. maiden name). Some states, such as New York, also allow married couples to adopt any new surname upon marriage, which may be a hyphenated form of the bride and groom, a combination of parts of their family names or any new family name they can agree upon adopting as the married name.
In order to maintain one's identity, it is desirable to obtain a formal order so there is continuity of personal records. The open and notorious use of a name is often sufficient to allow one to use an assumed name. In some jurisdictions, individuals may register the use of a trade name that is distinct from their legal name and is registered with the county clerk, secretary of state or other similar government authority. Individuals who wish to publish materials and not have the publication associated with them may publish under a pseudonym; such right is protected under the US Constitution's guarantee of freedom of speech.
 United Kingdom
In the different parts of the United Kingdom, legally speaking, the only generally accessible way to change one's name is by usage and repute. Once you generally use a name and get to be known under this name, it will be your legal name, provided that you do not change your name for fraudulent purposes. (In theory a name change by Act of Parliament is also possible and names can be changed by royal licence, but this latter possibility is only used in cases involving nobility.)
In practice, though, one of two documents are necessary to have one's name changed on the files of government bodies and other institutions like e.g. banks: a deed poll or a statutory declaration.
The most commonly used instrument is the deed poll, a legal contract binding only a single person -- a Deed of Change of Name. The deed consists of three declarations, committing the person to abandoning their former name, using only the new name and requiring everyone else to use only the new name.
The deed is executed by signing, dating and a witness signature. A person's name can be changed as often and whenever they like provided it is not for the purposes of fraud or to deceive. The deed poll is typically prepared by a solicitor, but that is not a condition and it can be witnessed by anyone having sufficient standing to be credible.
The deed poll as such is incapable of legally changing a name -- only usage and repute brings that about. In practice it is accepted as evidence of a change of name, however, as it creates a situation that will normally lead to a legal name change, all the more so as the deed poll is in fact sufficient to have one's driving licence, passport, bank card etc. changed immediately -- and once all those documents are changed, there will be "usage and repute".
Except for children who are not older than one year and special cases like re-registration to enter the father's details, adoption or gender reassignment, a name change cannot be recorded in the birth register in England and Wales. Birth certificates will thus continue to indicate the original name only.
There is no legal requirement to list the name change with any group or body, although it can be entered in the Enrolment Books at the Central Office of the Supreme Court of Deeds. The name change only operates for the future. Previous time-dependent legal documents, certificates etc. retain the prior name.
In Scotland and Northern Ireland a name change can optionally be recorded on the birth register, but evidence (like a driving licence, bank statements, passport, membership cards etc.) is needed to show usage of the new name over an extended period of time. No special process is required, but the most convenient way to obtain this evidence is of course to execute a deed poll anyhow. There are restrictions on the number of name changes that can be registered, but there is no restriction on the number of times a person can legally change their name. Where a name change is recorded, birth certificates will show the new name and the original name.
Women can in practice have their surname changed to their husband's name on marriage or back to a former name on divorce without a deed poll. Driving licences, passports etc. are routinely changed on simple request and presentation of the marriage certificate or the divorce decree -- and the changed documents then bring about usage and repute.
The restrictions on names prevent the acceptance of those containing numbers or non-alphabetic symbols and, probably, the acceptance of a single name or one that is impossible to pronounce. At least, il will be practically impossible to create "usage and repute" for such a name, as it will not be accepted. Theoretically, an inherited title (Sir, Dame, Lord, Lady, Baron, Baroness, Count, Earl, Countess, Marquess, Marchioness, Duke, Duchess, Prince, Princess, King or Queen) as a first name is possible, but there almost necessarily is a fraudulent purpose then, that prevents a legal name change.
Famous instances of people renamed by deed poll include Screaming Lord Sutch, 3rd Earl of Harrow, the founder of the Official Monster Raving Loony Party and Elton Hercules John, the singer, composer and musician, who was previously called Reginald Kenneth Dwight.
"Michael Howard of Leeds", according to the Guardian newspaper in November 1999, changed his name by deed poll to "Yorkshire Bank Plc are Fascist Bastards" after being charged £20 for a £10 overdraft. When the bank asked him to close his account, he asked them to repay the 69p balance with a cheque made out in his new name.
 Civil Law
Although as in other jurisdictions a citizen of Quebec may informally use whatever name he or she wants, procedures for formal name change are very strict as Quebec operates under a civil law system. The decision must be authorized by the Director of Civil Status, and requires a valid reason for changing the name, including long-term use of the new name (in the Montreuil case cited below, the Quebec appeals court has considered five years' use to be a sufficient reason), difficulty of use due to spelling or pronunciation, or bearing a name that another person has made infamous.
This has occasionally led to controversy. A lawyer named Micheline Montreuil, a non-operative male-to-female transsexual, had to undergo a lengthy process to have her name legally changed. Initially, the director of civil status refused to permit the change on the grounds that a male could not bear a female name. According to Quebec law, Ms Montreuil could not change her record of sex because this requires proof of a completed sex reassignment surgery, which she has not had. On November 1, 1999, the provincial court of appeal ruled that nothing in the law prevented a person who was legally male from legally adopting a woman's name. (Ms. Montreuil was initially prevented from changing her name despite this ruling on the grounds that she had not established general use, as normally required for a name change; the Quebec appeals court finally authorized the change on November 7, 2002.) 
 Name change on religious conversion
Adherents of various religions change their name upon conversion or confirmation. The name adopted may not have any legal status but will represent their adopted religious beliefs.
It has been historical Christian practice to adopt a name on baptism and/or confirmation. 
Jewish people customarily give their children two names: a secular name for everyday use and a Hebrew name for religious purposes. 
Reverts to Islamic faith may choose a new name. Boxer Cassius Clay's adoption of the name Muhammed Ali is a well-known example. 
Those who embrace the Hindu faith may choose a name. 
 See also
- Religious conversion
- Legal aspects of transsexualism for information about name change for transgender and transsexual people