Following is an abbreviated list:
Illegal Presence in the US
If you accumulate six months of unlawful presence in the US (by overstaying your visa or entering illegally in the first place), you will be barred from re-entry for three years. If you accumulate a year or more of illegal presence, you will be barred for 10 years. This bar can potentially be waived by filing Form I-601A if you have a qualifying relative who would suffer extreme hardship if you were barred from the US.
Certain Physical or Mental Disorders
You can be excluded for communicable diseases such as gonorrhea, syphilis, infectious leprosy, active tuberculosis and certain other diseases. HIV was removed from this list in 2010. You can also be excluded for failure to be vaccinated against communicable diseases. A mental disorder can result in a bar if you are likely to become a danger to yourself or others. This barrier to entry can be waived under certain circumstances.
A variety of criminal offenses can get you barred from the US, including:
You can be found inadmissible for drug abuse or addiction, or for drug trafficking. This ground of inadmissibility is not waivable. You can even be barred for simply admitting to US immigration officials that you use or have used drugs, even if you were never arrested or convicted. Ironically, you can be prevented from entering California for having used marijuana, even though recreational marijuana use is legal in California (it is still illegal under federal law).
Crimes involving ‘moral turpitude’
This category includes passing bad checks, theft, burglary, human trafficking, money laundering and many more crimes. Even these crimes can be excluded if the perpetrator was a minor at the time, only one offense was committed, and the crime was committed more than five years ago. Crimes such as DUI or “causing a disturbance” are not considered crimes of moral turpitude. Commission of a crime involving moral turpitude can be waived using Form I-601.
Multiple criminal convictions
The general standard for inadmissibility on this ground is conviction of two or more crimes to which you were sentenced to five or more years in prison. This ground of inadmissibility can be waived.
A variety of immigration crimes can result in a bar to entry into the US. Deportation followed by another illegal entry to the US, for example, will result in a bar that cannot be waived even by filing Form I-601 or Form I-601A, and can also result in prison time. Other immigration crimes, such as overstaying your visa, can potentially be waived using Form I-601A.
Prositution or Other Vices
You can be denied entry to the US for prostitution or another illegal commercialized vice such as gambling, or the intention to engage in such activities while in the US. Thus is determined by criminal convictions, a history of such activities, or a suspected intention to engage in such activities while in the US. A history of prostitution, without current intent, can perhaps be waived by filing Form I-601.
Espionage or Terrorism
Anyone associated with espionage or terrorism, or suspected of an intention of engaging in these activities in the US, is inadmissible, and this ground of inadmissibiity cannot be waived. Involvement with a terrorist organization such as ISIS, without committing an actual act of terrorism, is enough to trigger the bar. Even Nelson Mandela required special permission from the Secretary of State to enter the US. No waiver is available except from the Secretary of State.
Participation in political crimes of persecution
This category exludes former Nazi officials as well as people who participated in genocide, torture or the murder of dissidents or political opponents. You can also be excluded for severe violations of religious freedom while serving as a foreign official. This ground of inadmissibility cannot be waived.
Certain people who pose a threat to US foreign policy or who are members of a totalitarian political party can be excluded. Technically, membership in the Communist Party can get you barred, but this ground of inadmissibility is full of exceptions, as evidenced by the number of Chinese students and tourists who regularly spend time in the US.
Persons ‘likely to become a public charge’
A public charge is someone who relies on public assistance to support themselves — welfare payments, for example. This ground of inadmissibility applies mainly to family-based immigration petitions. Although this ground of inadmissibility cannot be waived through an I-601 application, the obvious solution is to find a sponsor with the financial means to support you.
Of course, if you seek entry to the US based on a job offer, you won’t need to worry about this grounds of inadmissibility — since employers are required by law to pay immigrants the same salary as they would pay a similarly qualified US worker, US immigration authorities realize that you are unlikely to become a public charge.
The foregoing represents only a partial list of the legal grounds for inadmissibility to the US. Unfortunately, space limitations prevent a full discussion of all possible grounds of inadmissibility. Consult an experienced immigration lawyer for further information.