The Minneapolis company said it immediately told authorities and financial institutions once it became aware of the breach and that it is teaming with a third-party forensics firm to investigate the matter.
Target Corp. said that customers who made purchases at its U.S. stores during the impacted period and suspected unauthorized activity should call them at 866-852-8680.
Experts say Target shoppers should take a close look at their credit and debit card statements for any suspicious transactions.
If something stands out, cardholders should check with anyone else who has access to the account, like a spouse, children or business partners.
If fraud is detected, cardholders need to report it immediately to avoid being held liable.
If you don't find suspicious transactions, it doesn't necessarily mean the account isn't compromised.
Experts say it could take months before any signs of fraud appear.
Cyber-crime experts say there are a lot of stolen credit cards out there right now, making it hard for thieves to sell them.