Ineligibility to enter the U.S. can be due to many reasons. Inadmissibility often stems from "conviction, or, admitting, even without conviction, the essential elements of a Crime Involving Moral Turpitude." Individuals who are 18 or older and have a criminal record for crimes of moral turpitude could be denied entry. The list of crimes that are considered immoral is quite extensive. Moral turpitude is a vague concept that can change over time. Generally, it refers to actions that shock the public conscience.
The most common crimes that involve moral turpitude and would make you inadmissible include murder, manslaughter, rape, theft, bribery, forgery, aggravated battery, prostitution, and fraud.
While not the most common, drug trafficking and possession are still offences that could deem someone inadmissible. Possession of a controlled substance conviction may cause a denial of entry.
At present, offences such as driving under the influence, breaking and entering, disorderly conduct, and simple assault do not render a person inadmissible to the U.S. Yet, if someone has multiple convictions for lesser offences or other misdemeanours, they could be denied entry. For those convicted of a lesser offence like possession of a controlled substance, they might risk attempting entry, hoping it goes unnoticed or take the direct approach and apply for a waiver that could potentially permit their entry into the US.
However it should be noted that applying for an inadmissibility waiver is a drawn-out and comprehensive process. For further information, the U.S. Customs and Border Protection website should be consulted.
A person could be deemed inadmissible if the U.S. government has a reason to believe they are or were a drug trafficker. Additionally, being a drug addict or abuser can also be grounds for inadmissibility. Inadmissibility can result from a conviction related to any controlled substance violation or from admitting (even without a conviction) the essential elements of a violation of a law pertaining to a controlled substance, as stated in INA 212. The U.S. government has the authority to bar anyone who has a possession conviction as they are considered a "drug abuser."
If an individual is of Aboriginal descent, they may not necessarily need an inadmissibility waiver despite having a criminal conviction. This is due to the 1849 Jay Treaty between the US and the UK (for Canada) which allows free border crossing for Aboriginal people. Under this treaty, those born in the US or Canada with at least 50% Aboriginal blood are allowed to enter, live in, and work in either country without facing immigration restrictions.
Even inadmissible individuals can be permitted entry for compelling reasons. They may be granted humanitarian entry to the United States for events such as funerals, deathbed visits, weddings of a close relative, or medical treatment. The decision to grant humanitarian entry is made by border security at the point of entry.