Target says it's fixed the problem that allowed credit and debit card information on as many as 40 million accounts to be stolen. It says credit card holders can continue to shop at its stores.
But the chain also says customers should check their statements carefully for unauthorized charges.
Customers who see suspicious activity in their accounts are being told to call Target at 866-852-8680.
The accounts at risk are those of customers who swiped their cards at stores in the U.S. between November 27th and December 15th. The breach didn't affect online purchases. The stolen information included Target store brand cards and major card brands such as Visa and MasterCard.
Target isn't saying exactly how the data breach occurred. It says it's working with a third-party forensics firm to investigate it, and to prevent future breaches.
Target's stocked dipped about two percent in this morning's trading.