Identity theft occurs when someone obtains your personal information, such as your credit card data or Social Security number, to commit fraud or other crimes.

Guard your information online. These days, many of us do most of our shopping and banking on the web. With all those account numbers and passwords floating around, it’s easy for someone to nab your information and go on a spree.

• Clear your logins and passwords. This is especially important if you’ve been working on a public computer. Change logins and passwords monthly.

• Pay for online purchases with your credit card, which has better guarantees under federal law than your online payment services or your debit card.

• Be alert for phishing, a trick in which spam or pop-ups mimic legitimate banks or businesses to obtain your personal information, which they use to access your accounts. Always verify that you’re on a familiar Web site with security controls before entering personal data.

Monitor your bank and credit card statements. Check your accounts regularly so you know when something’s awry. Purchases you didn’t make should be obvious—like a gas fill-up halfway across the country.

Verify your mailing address with the post office and financial institutions. Identity thiefs may fill out change of address forms so that delinquent credit notices remain off your paper billing radar.

Monitor your credit report. By law, you’re entitled to a free report every year from each of the three bureaus (Equifax, Experian, and TransUnion). Request one every four months, changing bureaus each time. You can order the report directly through each agency, or at annualcreditreport.com. Scan it for abnormal activity, such as accounts or credit cards you didn’t open.

Shred sensitive documents. Buy a shredder and regularly shred outdated bank statements, credit card applications, bills, and anything with your personal information before tossing it into the trash or recycling. Junk mail often includes some of your personal details.

Fraud alerts. Some identity-theft protectors will immediately place fraud alerts on your files with the three main credit bureaus, whether you’ve been victimized or not. In essence, it forces any bank or credit agency to balk before approving credit requests in your name. It’s not foolproof, though. The law only requires the creditor to take reasonable precautions before extending credit. This may only be a speed bump for a practiced thief, so don’t consider it a guarantee that your identity won’t be swiped.

Credit freezes. Freezes are far more effective than alerts. Icing your files prevents any company from accessing your credit unless you already do business with them, effectively sealing your records against any new creditor. Freezes can be a pain if you’re seeking a mortgage or student loan—or any form of credit. You’ll have to contact the bureaus to unfreeze your records, which can take up to three days. Plus, the credit bureaus normally charge a small fee whenever you freeze and unfreeze your files. Credit freeze rules vary by state.

Alerts and freezes are two measures you can take yourself, so consider whether you want to pay a company to do it for you.

If you’ve detected fraudulent activity, notify the financial institution where the fraudulent activity occurred first so they can freeze your account. Depending on the situation, you’ll need to file a complaint with the FTC and your local police department, as well as investigate all of your other accounts. And keep an eye on your credit report.