Obtaining a U.S. visa does not mean that an alien will be automatically allowed to enter the country. A visa merely permits an alien to board transportation bound for the U.S. and subsequently apply for admission upon arrival. Admission is the lawful entry of an alien into the United States after inspection and authorization by an immigration officer. Upon arrival at a port of entry in the U.S., an alien will be inspected by a Customs and Border Patrol officer. The officer will inspect the alien's visa or advance parole documents. The officer may find that the alien is not admissible if the officer finds or suspects that the alien's intended purpose for entering the US is different from the authorized purpose of the visa. Moreover, an immigration officer may deny entry if the officer finds that the alien has committed unlawful activities in one or more of the following categories: crimes, immoral acts, violations of immigration law, ordered removals, failing to attend removal proceedings without reasonable cause, smuggling aliens into the U.S., misrepresentations, unlawful voting, international child abductions, avoiding a draft into the U.S. armed forces, or for public security concerns.
 
Crimes
An alien with a criminal record may be barred from admission into the U.S. Crimes of moral turpitude, drug offenses, multiple offenses, and persons who have engaged in prostitution or solicitation of prostitutes within the past ten years, will generally be considered ineligible for admission.
 
To be excludable for admission, an act must constitute a crime under the laws where it occurs.

•Crimes of moral turpitude
An alien who is convicted of, or who has admitted to, committing a crime of moral turpitude with a sentence of one year or longer may be barred from admission.
 
Because of the broad definition of crimes of moral turpitude, most crimes will fall under this category. Basically, any act which by itself is morally reprehensible and intrinsically wrong or evil by its nature will render such an act a crime of moral turpitude even without statutory prohibition.
 
Common examples of crimes involving moral turpitude include aggravated assault, sexual offenses, child abuse, aggravated driving while under the influence of drug or alcohol, illegal use of a credit card, stealing cellular air time, larceny, malicious trespass, willful tax evasion, possession of stolen mail, counterfeiting, and perjury.

•Drug offenses.
A drug offense for the purposes of U.S. immigration laws is considered broader than in other contexts. It includes violations of any law or regulation relating to a controlled substance, no matter whether it is federal, state or foreign, and the definition of an illegal drug may be broader than the one designated by the Attorney General. It covers persons with a past conviction or admission of an offense. It may include a person a USCIS officer knows or has reason to believe is a drug trafficker, or who may assist in illicit trafficking. Spouses and children of such persons may also be inadmissible if they have benefited from a drug offense within the past 5 years.
 
Even expungement of the criminal record in court will not bar the use of underlying facts to deny admission.

•Two or more convictions
An alien with two or more convictions where the aggregated confinement actually imposed is more than 5 years, even if the convictions are from a single trial, is inadmissible.

•Crimes of domestic violence, stalking, or violations of protective orders; crimes against children
An alien or a lawful permanent resident is subject to deportation and will be barred from readmission to the U.S. for the above crimes. A domestic relationship includes a current or former spouse and cohabiter living together as a spouse, etc.

•Immoral Acts
This term typically refers to the practices of polygamy and prostitution, but is subject to the officer’s discretion.
 
Violation of immigration laws
This category covers most violations for immigration purposes.

•Persons unlawfully present in the U.S.
A person who is unlawfully present in the U.S. for more than 180 days but less than 1 year and who voluntarily departed the U.S. before the commencement of removal proceedings will be barred for 3 years after their departure.
 
A person who has been unlawfully present in the U.S. for 1 year or more will be barred from reentry into the U.S. for 10 years from the date of departure or removal.

•Persons who have been ordered removed from the US
A person who has been ordered removed after April 1, 1997 will be barred from admission for 5 years from the date of such removal (or 20 years for a second or subsequent removal or at any time if the alien is convicted of an aggravated felony), unless prior permission is granted.

A person who departed the U.S. while an order of removal was outstanding after a deportation or removal hearing will be barred from admission for 10 years after the date of such alien's departure or removal (or 20 years from such date for a second or subsequent removal, or at any time if the alien is convicted of an aggravated felony), unless prior permission is granted.

•Persons who fail to attend removal proceeding without reasonable cause
An alien who without reasonable cause fails or refuses to attend or remain in his/her removal proceeding will be barred for 5 years from re-entry.

•Smuggling aliens into the U.S.
A person who at any time knowingly has encouraged, induced, assisted, abetted, or aided any other alien who was entering or trying to enter the U.S. in violation of immigration law is inadmissible.

•Persons subject to a final order
A person who has violated the U.S. Immigration and Nationality Act by falsely making, using, or assisting others in procuring fraudulent documents (such as someone else's lawful documents and false car registration) is inadmissible.

•Misrepresentation
Most alien sanction cases occur in the context of misrepresentation. A person, who by fraud or willfully misrepresenting a material fact, seeks to obtain a visa or other documents necessary to enter the U.S. or to obtain other benefits is inadmissible. For example, falsely claiming United States citizenship in order to obtain benefits from federal or state governments is cause for a bar from admission.

•Unlawful voting
A person, who has voted in violation of any federal, state, or local constitutional provision, statute, ordinance, or regulation, is inadmissible.

•International child abduction
A person who is not granted custody of a U.S. citizen child, and who detains, or retains the child or withholds the custody of the child outside the U.S. from a person awarded with custody of the child, is barred from admission. The bar applies to any other person who assists, supports, or provides safe harbor, as well as the spouse, children, parents, and siblings of the inadmissible alien.

•Draft evaders
Every male U.S. citizen born after 1959 is required to register for the draft when he turns age 18. A similar rule applies to certain lawful permanent residents. A person who departed or remained outside the U.S. to avoid or evade service in the U.S. military during war or a time of national emergency is barred from admission. This bar does not apply to an alien who at the time of departure was a nonimmigrant and who is seeking to reenter the U.S. as a nonimmigrant.

•Public safety grounds
A person who seeks to enter the U.S. for espionage, sabotage, illegal activity, or terrorist activity is barred from admission.