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Following people online, with cookies, tagged pixels and even voluntarily given information, has been a big business. Now much of the same technology is moving into the physical world.

A company called Euclid Analytics uses the Wi-Fi antennas inside stores to see how many people are coming into a store, how long they stay and even which aisles they walk. It does this by noting each smartphone that comes near the store, feeding on every signal ping the phone sends.

"Three years ago, I went to the Stanford mall in Palo Alto and could count about 30 percent of the people there" by listening for Wi-Fi signals, says Will Smith, the company's chief executive. "Now in San Francisco, it's about 60 percent. In Atlanta or Charlotte, it's 40 percent."

Using the information, retailers can tell whether someone walked by the store, whether a customer came in and how long the visit lasted. If it is a big store, with a couple of Wi-Fi antennas, the owner can start to see where in the store someone went.

Euclid is three years old and has about 100 customers, including Nordstrom and Home Depot. It has already tracked about 50 million devices in 4,000 locations.

The big initial use is the so-called bounce rate, or the percentage of people who come into the store who leave without making a purchase. But the technology also helps stores make sure that there is enough sales help or that enough registers are open. By seeing how people move in a store, retailers can also better determine where to place low-profit and high-profit items.

Mr. Smith says Euclid has more data than it gives to customers. It gives its customers only anonymous data in a collected form, so individuals won't be targeted. Stores using the technology may also put stickers in their windows telling customers they are being monitored and allowing them to opt out

It's likely, however, that over time Euclid and its partners could add an opt in feature, where people choose to be recognized, the way registered Amazon.com customers are greeted when they come to a site. Then people might be offered, say, free parking for staying 20 minutes in a store, or they could get a discount for visiting three times a month.

Computers are already recognizing people moving around, both voluntarily and involuntarily. Mr. Smith was talking about his company at a conference in Santa Monica, Calif., held by the Montgomery & Company investment firm. Down the hall, a company called Omnilink, which makes ankle devices for people under home arrest, talked about plans to expand into monitoring elders, children, workers on their own in the field and the infirm.

"Personal tracking is burgeoning," said Kelly Gay, Omnilink's chief executive. "It only takes one child to go missing or one person falling off a wall for us to see growth." The company has just started moving away from judicial monitoring, but it is seeing quick results: It tracks about 15,000 people on the judicial side (a very profitable business, with revenue of $140 a month a user), and it has 35,000 people who are voluntarily tracked.

Both are likely to increase, Ms. Gay says. On one side, prison overcrowding and tight budgets make it likely that more felons will do time outside the walls. On the personal side, the aging baby boom population makes it likely that there will be an increase in dementia, so people will require more monitoring.

But who needs ankle monitors when you have a smartphone? Ms. Gay's presentation was well received. Mr. Smith was mobbed with bankers willing to lend him money and entrepreneurs wanting to work with his monitoring business.