For a US citizen, a small crime may mean a fine. For a non-citizen, the same crime can lead to a canceled visa, a denied green card or citizenship, or removal (deportation). Immigration law and criminal law are separate, and immigration law is often stricter.
Stealing, even something small, can count as a crime involving moral turpitude (a serious category in immigration law). Stores call police and may also send a civil demand letter for money. Never "test" self-checkout or take something to "pay later".
A conviction for domestic violence, stalking, child abuse, or violating a protective order can make a non-citizen deportable, including a green card holder. Police in many states must arrest someone when they respond to a domestic violence call and see signs of violence. If you are a victim, see the guide on domestic violence help.
Drugs, including marijuana
Marijuana is legal for adults under state law in many states, but federal law still controls it. Immigration law is federal law.
Using, buying, possessing or working in the marijuana industry, even where it is legal in your state, can make you inadmissible, deportable, or unable to show good moral character for citizenship.
Admitting to a doctor or immigration officer that you used marijuana can be enough to cause problems, even without a conviction.
Do not carry marijuana products, including edibles and vapes, onto planes, across borders, or into federal property such as national parks.
Prescription medicines are legal only for the person named on the prescription.
Driving under the influence is a crime. For visa holders, a DUI arrest can lead the State Department to revoke the visa stamp, and may need a medical exam before a new visa. Repeated DUIs or DUI with injury can have worse effects. See the guide on DUI and your status.
Lying and false documents
Lying to an immigration officer or on an immigration form can make you permanently inadmissible.
Falsely claiming to be a US citizen, for a job, to vote, or on a form, usually has no waiver.
Using a fake ID, a fake Social Security number or someone else's work documents is a crime.
Immigration forms for visas, green cards and citizenship ask about arrests and charges, not only convictions. You must answer truthfully even if the case was dismissed or expunged (erased).
Keep a copy of the final court record for every case.
If you are charged with a crime
Tell your criminal lawyer or public defender you are not a US citizen and your exact status. They are required to advise you about immigration consequences of a plea.
Before you plead guilty to anything, ask an immigration lawyer too. A plea deal that sounds easy, like "just probation", can still have serious immigration effects.
Thinking state-legal marijuana is safe for immigrants.
Pleading guilty to "make it go away fast".
Forgetting an old arrest when filling in a form.