Here are some answers to the most common questions about the theft:
Q • I shopped at Target during that time. What should I do?
A • Check your credit card statements carefully. If you see suspicious charges, report the activity to your credit card companies and call Target at 866-852-8680. You can report cases of identity theft to law enforcement or the Federal Trade Commission.
You can get more information about identity theft on the FTC's website at www.consumer.gov/idtheft, or by calling the FTC, at (877) IDTHEFT (438-4338).
Q • How did the breach occur?
A • Target isn't saying how it happened. Industry experts note that companies such as Target spend millions of dollars each year on credit card security, making a theft of this magnitude particularly alarming.
Experts disagree about how the breach might have happened.
Avivah Litan, a security analyst with Gartner Research, says given all the security, she believes the breach may have been an inside job.
But thefts of this size are too big to be the work of company employees, says Ken Stasiak, founder and CEO of Secure State, a Cleveland-based information security firm that investigates data breaches like this one. Stasiak says that such breaches are generally perpetrated by organized crime or an overseas, state-sponsored hacker group.
Stasiak's theory is that the hackers were able to breach Target's main information hub and then wrote a code that gave them access to the company's point of sale system and all of its cash registers. That access allowed the hackers to capture the data from shoppers' cards as they were swiped.
James Lyne, global head of security research for the computer security firm Sophos, says something clearly went wrong with Target's security measures.
"Forty million cards stolen really shows a substantial security failure," he says. "This shouldn't have happened."
Q • Who pays if there are fraudulent charges on my account?
A • The good news is in most cases consumers aren't on the hook for fraudulent charges.