How do balance transfer cards work, and what are the fees and pitfalls?

A balance transfer card lets you move existing high-rate debt onto a new card offering 0% APR for an introductory window, usually 12–21 months. You pay no interest during that period, so 100% of your payment reduces principal. The catch is a transfer fee, typically 3–5% of the amount moved – transferring $10,000 can cost $300–$500 up front. The biggest pitfalls: the rate jumps to 20%+ when the promo ends, so you must clear the balance before then; new purchases may not get 0%; and a missed payment can void the promo entirely. You also need good credit to qualify, and the issuer may approve less than you owe. Make a plan to pay the balance off within the promo window, and stop charging the old card. A windfall or steady extra payment makes this strategy work.