Inadmissibility
Before being granted an immigrant or nonimmigrant visa, every alien must prove that s/he is not "inadmissible" under the U.S. immigration laws.  Inadmissibility is used to describe a vast array of conditions or characteristics which Congress has determined are undesirable, all of which are generally based upon the alien's conduct or status.  An alien who is deemed inadmissible will be denied admission into the U.S. even though s/he otherwise qualifies for a visa under a particular visa category.

While the list of grounds of inadmissibility under the U.S. immigration laws is quite extensive, most of them fall under categories relating to criminal violations, national security violations, immigration law violations, health related problems,  and financial problems experienced by the alien applicant.

Even if the INS makes a determination that an alien applicant is inadmissible, s/he may be eligible for a "waiver" of inadmissibility.  Where a waiver is granted, the INS, in effect, overlooks or excuses the ground of inadmissibility and grants the visa or permits the alien to adjust his or her status anyway.  Whether a waiver is available or whether it is ultimately granted to a particular alien applicant will depend upon the specific ground of inadmissibility with which the alien is charged and the specific factors and circumstances which the alien can prove in support of the waiver application (ie. that the alien has family ties in the U.S. or that it would be in the public or humanitarian interest of the U.S. to grant a waiver).