Online Ads Can Now Follow You Home

Advertisers already know what people are up to on their personal computers. But understanding their online whereabouts on smartphones or tablets has remained elusive.

A number of companies are trying to better pinpoint mobile users' online activity with new software and techniques they say could help advertisers track users across devices. By harvesting cross-screen identities, the ad industry could serve ads to mobile phones based on the interests people express when surfing the Web on their PCs. The emergence of cross-screen marketing is one of several new forms of technology aimed at solving a fundamental problem with mobile ads: It is harder to target people on smartphones than on PCs.