The clash between Massachusetts Sen. Elizabeth Warren and President Donald Trump over her Native American heritage highlights the varying methods tribes use to decide who belongs.
The decision has wide-ranging consequences for Native American communities and their relationship with the federal government.
Some tribes rely on blood relationships to confer membership. Historically, they took a broader view that included non-biological connections and people's value in society.
The 573 federally recognized tribes are sovereign governments that must be consulted on issues that affect them. Within tribes, enrollment also means being able to seek office, vote in tribal elections and secure property rights.
For centuries, a person's percentage of Native American blood had nothing to do with determining who was a tribal member. And for some tribes, it still doesn't.