Editor’s note: Previous versions of this story said the three- or four-digit security code typically printed on the back of a credit or debit card had been stolen in the security breach. According to Target, that is not the case.

Q: Who should be concerned?

A: If you shopped at a Target store and used any credit or debit card there between Nov. 27 (the day before Thanksgiving) and Dec. 15, your card data may have been stolen. That’s 40 million transactions, Target said.

Q: What about purchases online?

A: The crime appears to have been focused on the card being swiped in stores. Online purchases evidently were not affected. Neither were purchases in Target’s Canadian stores.

Q: What information did the thieves get?

A: Target said thieves have stolen the customer’s name, the credit card number and the card’s expiration date.

Q: Many people use Target debit cards linked to their checking account. What should they do?

A: Target says there’s “no evidence” that PINs — personal identification numbers — were stolen. But Target is advising all cardholders to be vigilant, monitor their accounts and report any suspicious activity. In addition, some outside security experts advise changing your PIN, to be safe.

Q: Which credit cards were involved?

A: All types of credit cards that were used at a Target store during that 19-day period are at risk.

Q: What are the risks with credit cards?

A: Visa, MasterCard and other major credit card issuers all have a zero-liability policy in such cases of fraud and criminal activity, which means customers can’t be charged if a stranger illegally makes a charge on their account. But credit card companies urge customers to check their statements and report any suspicious activity immediately.

Q: Will I need a new credit card?

A: Target’s response: “Right now, there’s no immediate need to have your cards reissued,” said Target spokesman Eric Hausman. “Again, monitor your account and report any suspicious activity.” Some outside security experts, however, urge greater caution. Karen Cassella, executive vice president of CBIZ Security & Advisory Services, suggests canceling any credit and debit card immediately if you’re concerned the information has been compromised. She also urges talking with your bank to see what it can do.

Q: When did Target first discover the problem?

A: Hausman said Target first uncovered a problem Sunday. News of the breach became public three days later, and Target confirmed it Thursday.

Q: Who is responsible for this crime?

A: “It’s obviously a sophisticated crime, but it’s an ongoing investigation, so it’s inappropriate to comment beyond that,” Hausman said.

Q: What could thieves do with 40 million credit and debit card numbers?

A: Brian Krebs, the tech blogger who first reported the breach, said thieves likely would try to sell chunks of the data to black-market data vendors. He also said the price it fetched would be low, because the market for stolen data is saturated. He said it could take weeks or months for any fraudulent charges to appear, if at all.

Q: What’s the likelihood that fraudulent charges will appear on my credit card?

A: Visa said that “because of advanced fraud-monitoring capabilities, the incidence of fraud involving compromised accounts is actually rare, and Visa fraud rates remain near historic lows.” Target isn’t the first retailer to face this issue.

Q: Does Target know of any cases where these compromised cards have been used for fraud?

A: “That’s an ongoing investigation. I can’t comment,” Hausman said.

Q: Is it safe to use my card at Target?

A: “The answer is yes,” Hausman said. Target officials say they’ve “identified and resolved the issue.”

Q: Who can I call to report suspicious activity on my account?

A: Call Target at 866-852-8680 or call the card issuer’s number printed on the back of the card.

Q: I’ve tried calling the Target number, and the line is busy.

A: On Thursday morning, Target’s lines were overwhelmed with concerned callers. Said Target spokesman Hausman: “We are experiencing significantly higher volume than normal.” He added that Target is scrambling to “add team-member support and capacity” and is expanding that “hour by hour.”

Q: Is the REDcard website working?

A: The Target REDcard site was flooded with concerned shoppers Thursday, and appeared to be working intermittently.

For more information, go to yourtechweblog.com.

— Tom Webb, Nick Woltman and Julio Ojeda-Zapata