By PHYLLIS KORKKI

YOU looked wonderful on your résumé. Your references raved about you. The interview went swimmingly. Yet you didn’t get the job. Oh, no: did they see that Facebook photo of you dancing on a table? Or find out that you’re six months behind on your mortgage payment?

You may never know why you weren’t hired, but be aware that background checks can make or break a job application. And in a data-rich world, the person with the fewest red flags may get the job.

Little hard research has been done on how hiring managers use the Internet to vet applicants. But you should assume that they are at least looking you up on search engines. So it’s wise to review the results of a quick search of your name.

It is very hard to remove anything questionable about yourself from a search engine, but you can at least push it lower by adding positive entries, said Barbara Safani, owner of Career Solvers, a career management business in New York.

Ms. Safani says she aims to help clients create a positive professional identity on the Internet through Google profiles, LinkedIn and ZoomInfo, for example, as these tend to be among the first to appear in search results. Adding such entries can also help people who have little or no presence online, as that can be viewed with suspicion these days, she said.

Job seekers should also give their Facebook page a close look. “How private is your Facebook page, really?” said Lewis Maltby, founder of the National Workrights Institute, an advocacy group. Despite privacy settings, he said, it’s not inconceivable that a potential employer could become a friend of one of your friends and thereby gain access to your page.

If you are showing or saying anything on Facebook that you wouldn’t want your grandmother to see, “take it down, now,” Mr. Maltby said. The same goes for friends’ posts that mention or “tag” you.

Mr. Maltby said he started his group “to extend protection for civil liberties to the world of employment because it doesn’t exist there.” But, he said, he also once ran a human resources department, and “you can’t blame H.R. people for looking at social network sites because hiring the wrong person is a very expensive mistake,” he said.

His concern is that some hiring managers may be disqualifying candidates “for ridiculous reasons that have nothing to do with the job” — for example, pictures of them drinking beer.

Chances are that employers wouldn’t tell you that a Facebook picture of you with a lampshade on your head was the reason you weren’t hired. But even if they did, they can generally refuse to hire you for any reason that isn’t specifically excluded by federal or state law, Mr. Maltby said. Such reasons include race, religion, disability or age.

And other online dangers may be lurking. You may continually be dropped from contention for jobs because of something about you on Internet databases, said Michael Fertik, founder and chief executive of ReputationDefender. His company, in Redwood, Calif., offers services aimed at helping people improve their online profiles and maintain their privacy. “Do not mistake the fact that you’re a decent person for the notion that you’ll look that way online,” Mr. Fertik said.

Without question, more data than ever is available on individuals. Hiring managers, if they were so inclined, might be able to learn about your political leanings, buying habits, hobbies and interests. But would they bother to do so? And would they hold that against you even if it had no bearing on the job?

Mike Aitken, director of government affairs at the Society for Human Resource Management, a professional association, is skeptical that many hiring managers have the time or inclination to investigate applicants on the Internet in any serious way — except, perhaps, when filling very high-level positions. Employers could even open themselves up to litigation by doing so, because candidates might contend that they were rejected for a job for a reason like religion that was revealed on a social network, he said. He also noted that such searches are costly.

Most employers contract background checks to other companies, he said, and these usually focus on things like whether a résumé is accurate, Mr. Aitken said. For certain positions, employers may also perform criminal and credit checks, he said.

Employers must obtain your permission to check your credit, but turning them down doesn’t bode well for your application, Mr. Maltby said. And if the employer rejects you for the job, offering some credible reason, he said, who’s to say it wasn’t actually because of the credit report? That’s a strong argument for checking your credit report for mistakes, and developing a good explanation if your credit score is genuinely poor (as it may well be if you’ve been out of work for a while).

Clearly, it can’t hurt to do a little research on yourself before sending applications. What you learn may surprise you. As Ms. Safani said, “Taking control of the situation is always a better strategy than sitting back and seeing what they find.”