"Congress has broad plenary authority to determine classes of aliens who may be admitted into the United States and the grounds for which they may be removed. Pursuant to the Immigration and Nationality Act (INA), as amended, certain conduct may either disqualify an alien from entering the United States ('inadmissibility') or provide grounds for his or her removal/deportation. Prominently included among this conduct is criminal activity. 'Criminal activity' comprises acts violative of federal, state, or, in many cases, foreign criminal law. It does not cover violations of the INA that are not crimes, most notably, being in the U.S. without legal permission. In certain circumstances, grounds for inadmissibility or deportability may be waived. Additionally, aliens facing removal may be allowed to remain in the United States, subject to discretionary or mandatory relief from removal for humanitarian reasons, such as through asylum, withholding of removal, or cancellation of removal. Aliens facing removal may also be permitted to depart the United States voluntarily, and thereby avoid the potential stigma and legal consequences of forced removal. Criminal conduct may effect an alien's eligibility for either voluntary departure or discretionary relief from removal. Additionally, criminal conduct is a key disqualifying factor under the character requirement for naturalization."
CRS Report for Congress, RL32480