Who can vote in Open Primaries?

State laws determine how primaries are run. In general, an open primary is one in which any registered voter can vote. A closed primary is one which is only open to registered voters of that party. Closed primaries are typically found in states where citizens, when registering to vote, are asked to declare a party affiliation. That affiliation goes into the voter registration records, and poll workers then use it to determine whether the voter is authorized to vote in the primary. If you're registered as a Democrat, you receive the ballot of the Democratic party, etc. Some state laws may permit a registered Independent to vote in the primary of either party. Obviously, if your state does not require party declaration upon registration, it won't have a truly closed primary, so will probably have some version of an open primary.