Every time Megan Ferington checks her Yahoo! e-mail, an ad for the wedding site The Knot pops up. "How do they even know?" the bride-to-be asked with a laugh.

It could be an example of online behavioral ad targeting, a strategy of tracking what people do, where they go and then presenting them with ads targeted to their interests. Yet, despite advertisers' growing interest in the technique, sometimes it can be hard to tell behavioral spots from ones with a rather less-focused approach.

In August, a mother who bought a birthday card for her daughter at the American Greetings store in South Hills Village was disconcerted to open her bag later and discover a brochure for a vaccine. The cover said, "Your daughter could become 1 less life affected by cervical cancer."

That was not behavioral targeting, said Ms. Ferington, who handles media inquiries for Cleveland-based American Greetings. Instead, the Gardasil brochures were delivered as part of a partnership between the card company and drug maker Merck & Co.

"Somebody who doesn't even have kids would have received it," said Ms. Ferington.

In that case, the ad might have had less impact, which is one reason so much attention is being given to new tools that target marketing messages to consumers based on their behavior, especially online where users leave a trail as they move around.

Online consulting firm eMarketer projects $1 billion will be spent next year on behaviorally targeted online ad spending. That compares to $220 million in 2005 and $3.8 billion projected for 2011.

In the past few months, AOL picked up behavioral targeting firm Tacoda; Yahoo! purchased Blue Lithium; even social-networking site Facebook made news with reports that it plans to allow marketers to target ads to its customers.

Anyone who has done a Google search and seen related ads pop up on the side has been targeted by behavior. Amazon.com shoppers expect to be offered additional choices produced by a computer that crunches information on past purchases by that individual and other customers.

An advertiser trying to target car buyers might go to a company such as Tacoda, for example, because each month it tracks unique visitors who research cars on sites in its advertising network, such as Cars.com, TheAutoChannel.com and the Kelley Blue Book site, kbb.com.

Those visitors, which the company this summer reported totaled more than 12 million, could be served targeted ads either at auto sites or anywhere else they might wander in the company's 4,000-site participating network. An ad even could appear next to a corn chowder recipe on the FoodNetwork.com or a USAToday.com story about Congress.

"It's not about relevant content but rather relevant audience," said Rick Gardinier, managing director of Downtown ad agency Blattner Brunner's bbdigital division.

There are issues to be worked out before behavioral ads become a must-have piece of an ad campaign.

On the marketer's side, it isn't always easy to understand why people behave the way they do and what ads will seem most relevant to them. As an advertising tool, it's also new enough that eMarketer reports only 34 percent of sites support behavioral targeting now, up from 29 percent last year.

Consumers, meanwhile, may struggle with relationship dilemmas. Like living in a small town, it's great to be recognized by the grocer, but there are times that being incognito is better. Sometimes the marketing might seem a little too personal, as when the mother found the brochure for a vaccine against cervical cancer and genital warts.

"In touchy areas, people don't want to feel they've been observed," said David Hallerman, senior analyst with eMarketer. Even for products like athlete's foot cream, consumers value privacy, he said. Advertisers will need to learn where those lines are and how to avoid crossing them.

Overall, research seems to show consumers are OK with a certain level of targeting. If they have to see ads, at least they see some for products that interest them.

So what interests them? The science of tracking Internet movement gives advertisers tons of data on where people are going online, but there's an art to figuring out its meaning.

It may seem obvious that someone who goes to a travel site for beach vacations would be receptive to seeing beach-related ads when he heads off to check flights. Yet the message could just fade into the background, said Ed Bardwell, chief technology innovations officer for Station Square-based ad agency Marc USA.

It might be more effective to send out a commercial for a ski resort that says, "It's hotter on the slopes," he said.

Cost comparisons can be tricky. Blattner Brunner ran a comparison in which the same ad was placed with and without using a Yahoo! behavioral targeting tool. The basic price per thousand views for the traditional version was almost half that of the behavioral version. The traditional effort drew about 900 leads at a cost of almost $120 per lead, while the behavioral one produced 625 leads at just under $80 per lead.

That appears to prove behavioral is more effective, said Mr. Gardinier, but there are reasons not to rely just on that tool. In addition to the limited number of sites that use the tool, some ad campaigns are meant to raise awareness among a broad group of consumers rather than target a narrow niche. And there are other online tools to try, such as buying space next to stories on a related subject.

Consumers can mess up behavioral targeting a bit by sharing a single computer within a family or deleting the so-called cookies that Web sites use to recognize a frequent visitor. Mr. Gardinier said only 15 to 20 percent of users regularly clear those out, while the rest either don't know how or don't mind that they are there.

Even if they don't do anything to block the Internet observers, it can be difficult to sort the behavioral ads from all the others out there. Mr. Bardwell noted, "Behavioral advertising is still very much in its infancy."