eMarketer estimates that marketers will spend $1.2 billion on behavior-targeted online advertising in 2006 - and forecasts that the market will surge to surpass $2 billion in 2008, reaching $2.1 billion, after having reached $1.5 billion in 2007, Three factors will contribute to those gains, eMarketer says: Behavioral targeting helps marketers get better results from fewer impressions; publishers are keen that behaviorally targeted ads tend deliver more revenue from secondary pages; users tend to find ads targeted by their actions to be more relevant to their needs.

Only 28.7 percent of respondents in a December American Association of Advertising Agencies survey said untargeted, mass-audience advertising would be "very effective" by the end of the current decade. Which is probably why, according to the American Advertising Federation, 84 percent of U.S. advertising professionals cite online advertising's ability to target fragmented audiences as an advantage.

"While digital ad targeting methods all have their analog counterparts, the interactive and dynamic tools that the Internet adds to the game give targeting a new face," according to David Hallerman, eMarketer senior analyst and the author of a new report, "Online Ad Targeting: Engaging the Audience."