How do balance transfer credit cards work?

A balance transfer card lets you move high-interest debt to a new card with a 0% introductory APR — typically for 12 to 21 months. You pay a transfer fee (usually 3–5% of the balance). During the 0% period, all payments reduce principal instead of paying interest, potentially saving hundreds or thousands of dollars. The strategy only works if you pay off the balance before the promotional period ends.