Someone can be claimed as your dependent if s/he meets all five of the following tests:

Test 1 - Relationship
The person must be your relative, either by blood or in law.  A person who lived in your home as a family member for the entire year can also be considered a dependent, as long as the relationship does not violate local law.
Test 2 - Marriage
The person cannot be married and file a joint return with her/his spouse.
Test 3 - Citizenship
The person must be one of the following:
    * a U.S. citizen or resident alien, or
    * a resident of Canada or Mexico, or
    * your adopted child who is not a U.S. citizen, but who lived with you all year in a foreign country.
Test 4 - Income
Generally, the person's gross income must be less than $2,500.  Gross income does not include nontaxable income, such as welfare benefits or nontaxable social security benefits (like SSI).
If the person is your child, s/he can have a gross income of $2,500 or more if:
    * s/he was under age 19 at the end of the year,
    * s/he was under age 24 at the end of the year and was a full-time student for at least 5 months.
Test 5 - Support
You must have provided over half of the person's total support (food, shelter, clothing, medical care, education) for the year.

Note:  A person you claim as a dependent cannot be claimed as a dependent on anyone else's return.

1.  Is your daughter, son, adopted child, grandchild, step child, or foster child, AND
2.  Was (at the end of the tax year)
    * under age 19,
    * under age 24 and a full-time student, or
    * any age and permanently and totally disabled, AND
3.  Either lived with you in the United States for more than half of the tax year (for the whole year if a foster child) OR was born or died in the tax year and your home was the child's home for the entire time s/he was alive during the year.