A U.S. Citizen can petition to bring his/her fiancée to permanently live in the United States. There are specific eligibility requirements to bring a fiancée to reside in the U.S. If you and your fiancée meet the eligibility requirements, a petition is filed with the United States Citizenship and Immigration Service (USCIS) in the U.S., and after it is approved, your fiancée will obtain the K-1 Visa stamped onto her/his passport at the U.S. Embassy/Consulate where she/he resides, after a background check, a medical exam, and an interview process.
Once the fiancée enters the country, the marriage must occur within 90 days of the arrival date. If the marriage does not occur within the aforementioned time, your fiancée will not have legal status to remain in the United States beyond the 90-day-period for the K-1 Visa serves no other purpose other than marriage to the USC Petitioner.
Amongst some of the eligibility requirements, the parties must be unmarried or duly divorced; the parties must have met in person within the two years prior to the filing of the petition (this requirement can be waived but only under very specific circumstances); the USC Petitioner must satisfy certain financial requirements to be qualified to sponsor his/her fiancée - a requisite that is generally satisfied by demonstrating that the Petitioner earns at least 125% of governmentally defined poverty guidelines for the household size in question.
The K-1 fiancée visa is temporary but once married, your spouse can immediately apply for permanent residence in the U.S. The U.S. Petitioner can also petition for his/her fiancée's unmarried child(ren) under 21 years old to also immigrate to the U.S. under a K-2 visa.
Bringing your fiancée to permanently reside in the United States involves a multi-step process. Several government agencies are involved in the approval including the USCIS, the State Department's National Visa Center, and later, the U.S. Embassy or Consulate of your fiancée's country. This process normally takes several months until the Visa is issued by the U.S. Embassy or Consulate of your fiancée's country.