The eligibility criteria for the Green Card are very specific and depend on whether the applicant belongs to an immigrant category described in the Immigration and Nationality Act (INA). 

Family-based immigrants fall into one of two major categories, immediate relative or family preference.

Immediate Relatives: 
U.S. citizens may require for certain family members to receive a Green Card:
•Spouse of a U.S. citizen;
•Unmarried child under 21 years old of a U.S. citizen, and 
•Parent of a U.S. citizen (if the U.S. citizen is 21 years old or older)

Family “preference immigrant” categories:
•First preference (F1) - unmarried sons and daughters (21 years old and older) of U.S. citizens;
•Second preference (F2A) - spouses and children (unmarried and under 21 years old) of lawful permanent residents;
•Second preference (F2B) - unmarried sons and daughters (21 years old and older) of lawful permanent residents;
•Third preference (F3) - married sons and daughters of U.S. citizens; and
•Fourth preference (F4) - brothers and sisters of U.S. citizens (if the U.S. citizen is 21 years old and older).