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A K-1 Visa (also known as a Fiance(e) Visa) is a nonimmigrant visa Visa issued to the fiancé(e) of a United States citizen. The K-1 Visa allows the visa holder to enter the United States for up to 90 days during which time they may get married and file for Adjustment of Status (to become a Legal Permanent Resident of the US). After filing for Adjustment of Status they may reside in the US beyond the 90 day K-1 Visa validity while their Adjustment of Status application is processed. While the K-1 Visa is legally classified as a non-immigrant visa, it usually leads to important immigration benefits and is therefore often processed by the Immigrant Visa section of United States Embassies and Consulates worldwide.
Who is Eligible
U.S. citizens who will be getting married to a foreign national in the United States may petition for a fiancé(e) classification (K-1) for their fiancé(e). You and your fiancé(e) must be free to marry. This means that both of you are unmarried, or that any previous marriages have ended through divorce, annulment or death. You must also have met with your fiancé(e) in person within the last two years prior beginning the K-1 Visa petition process (filing the I-129F). This requirement can be waived only if meeting the non US Citizen's fiancé(e) in person would violate long-established customs, or if it would create extreme hardship on the US Citizen.
Applying for a K-1 Visa
An American begins the process of petitioning a foreign fiancé(e) by filing a form I-129F with US Citizenship and Immigration Services. Official instructions are available from the US Citizenship and Immigration Services web site, or the Department of State web site. No attorney or other intermediary is necessary to petition someone for a K1 visa, however some choose to hire an attorney to handle the paperwork and facilitate the process.
Once a visa petition has been filed, the USCIS service center sends it to the National Visa Center, which in turn forwards it to the Embassy or Consulate nearest the foreign fiancé(e). The Embassy or Consulate will notify the foreign fiancé(e) that a petition has been received, and provide information about how to schedule a medical exam with the panel physician as well as how to schedule an appointment for a visa interview at the Embassy.
At the visa interview a Consular officer reviews the documents that both the American petitioner and the foreign fiancé(e) have submitted. The Consular officer looks for evidence of a relationship, and asks the foreign fiancé(e) questions to determine that a bona fide relationship exists. Typically, Embassies request that fiancé(e)s bring evidence of their relationship to the interview, such as photographs together, correspondence between the two, evidence of remittances to the fiancé(e), phone bills showing calls to each other, etc.
Once the interview is finished the Consular officer can issue the visa, if he is convinced of a bona fide relationship that meets all legal requirements. The officer may also request that further evidence be submitted before making a decision. If the Consular officer does not think the relationship is bona fide or finds some legal impediment to issuing the visa, the petition will be returned to USCIS and recommended for revocation and no visa will be issued.
A K1 visa is printed on a self adhesive label, similar to a tourist visa, but annotated with the name of the petitioner. The label is placed into the fiancé(e)'s passport. It is valid for one entry into the United States for six months from the date of issuance.
The total time from filing of the initial petition to the actual issuance of a visa can vary. You can check the current processing times in the Timeline section. The length of time depends partly on the processing speed of the USCIS center that receives the visa peition, as well as availability of appointments at the Embassy or Consulate nearest to the foreign fiancé(e). Visa petitioners and their fiancé(e) can avoid unnecessary delays by making sure their applications are completely and accurately filled out, and by scheduling necessary appointments with the panel physician and US Embassy as soon as they are eligible to do so.
Note: The US Citizen may also apply to bring the K-1 Visa holder's unmarried children, who are under age 21, to the United States.
Please see the guide on how to file for a a K-1 visa: Step-by-Step Guide on How to File for a K-1 Visa for your Foreign Fiance(e)
After the K-1 Visa is Approved
IMPORTANT: BE SURE TO CHECK OUT THIS HORROR STORY  DON'T LET THIS HAPPEN TO YOU!
Upon entering the US on a K-1 Visa, the visa holder must get married to the original I-129F sponsor (for the K-1 Visa) within 90 days. If the marriage does not take place within 90 days or the K-1 Visa holder marries someone other than the original US Citizen who petitioned for them, the K-1 Visa holder will be required to leave the United States. Until the marriage takes place, the K-1 visa holder is considered a non-immigrant. A non-immigrant is a foreign national seeking to temporarily enter the United States for a specific purpose. A K-1 Visa holder not obtain an extension of the 90-day original
If the K-1 Visa holder intends to live and work permanently in the United States they should apply to become a permanent resident after the marriage. Please note, K-1 Visa holder will receive conditional permanent residence status for two years. Conditional permanent residency is granted when the marriage is less than two years old at the time the Adjustment of Status application is processed.
Note: A K-1 Visa holder may enter the United States only one time with their visa. If the K-1 Visa holder leaves the country before they are married, they may not be allowed back into the United States without a new visa. Also, do not leave the U.S.A. while the adjustment of status is pending, as doing so is considered abandoning the petition for adjustment of status, requiring that you start over; unless you get a special stamp in your passport to say otherwise.