U.S. governmental regulatory agencies require that credit unions restrict their membership to defined segments of the population, such as people who live, work, worship, or attend school in a well-defined geographic area; employees of specific companies or trades; members of specific non-profit groups (alumni associations, conservation or other advocacy organizations, lodges, churches, or the like); or a particular occupational group (teachers, doctors, etc.).  In the U.S., this is referred to as a credit union's "field of membership."

Credit unions generally follow the principle of "once a member, always a member," which allows current credit union membership to continue even if the individual would no longer qualify to be a member (such as having changed professions or moved outside the area). However, many credit unions reserve the right of expulsion against a member who causes a financial loss.

If a member voluntarily terminates their membership, they may or may not be eligible to rejoin, depending on the credit union's policies and government regulations.

Credit unions may typically be chartered to serve a specific employee or associational group or groups (often called a Select Employee Group or "SEG Charter"), all members of a trade, industry, or profession (a "TIP Charter"), or have a "Community Charter" (typically a field of membership of anyone who lives, works, goes to school, or attends religious services in a particular city, county, or counties).

Typically, members' families -- such as immediate family or household members -- can also join the credit union.  In the United States, the National Credit Union Administration or a state regulator -- depending upon whether or not the credit union is chartered by the federal government or by a state -- decides whether or not to approve or deny proposed field of membership.

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