Ad Blockers, Internet Advertisers Play Cat and Mouse

A cat-and-mouse game between ad blockers and firms hired to circumvent them is playing out across the Internet, as ad revenue moves online.

For years, users fed up with banner and pop-ads have been downloading software to block them from their PCs and mobile devices. Ad-blocking firms and communities of developers arose to create and share often-free versions of ad-blocking software. But the ad-blocking deprives publishers and other ad sellers of a chunk of revenue. Research company eMarketer forecasts online-ad spending will reach $171 billion this year in the 22 big markets it tracks. That is up 18% from the year before and represents 30% of all ad spending for those countries.