GoogleGoogle reached out this evening to Web publishers using its advertising, warning them to review their privacy policies now that Google is about to track their users more closely.

The notification comes a day after Google announced it is starting to track users’ Web surfing preferences in order to target them with advertising that closely matches their tastes. Google reaches about 74 percent of U.S. Web users, according to Comscore, and so is collecting an immense amount of information when those users visit different sites.

To conduct its targeting, Google is dropping “cookies” on your browser each time you visit a Web page owned by a publisher that users Google advertising. Those cookies track you as you surf from page to page. If you tend to visit lots of sports pages, for example, Google classifies you as a “sports enthusiast” and chooses to serve up advertisement related to sports.

Google is essentially giving publishers a heads-up that they should tell their users about this new targeting by revising their privacy policies. (In separate but related story, the NYT’s Saul Hansell has an excellent review of how Google’s practices compare to those of other companies, including privacy policies. It’s too bad he doesn’t mention AOL’s Advertising.com, which reaches 91 percent of U.S. Web users, according to Comscore, and so has a greater reach than Google.). Here’s the message Google just sent to publishers:

Interest-based advertising will allow advertisers to show ads based on a user’s previous interactions with them, such as visits to advertiser website and also to reach users based on their interests (e.g. “sports enthusiast”). To develop interest categories, we will recognize the types of web pages users visit throughout the Google content network. As an example, if they visit a number of sports pages, we will add them to the “sports enthusiast” interest category. To learn more about your associated account settings, please visit the AdSense Help Center at http://www.google.com/adsense/support/bin/topic.py?topic=20310.

As a result of this announcement, your privacy policy will now need to reflect the use of interest-based advertising. Please review the information at https://www.google.com/adsense/support/bin/answer.py?answer=100557 to ensure that your site’s privacy policies are up-to-date, and make any necessary changes by April 8, 2009. Because publisher sites and laws vary across countries, we’re unfortunately unable to suggest specific privacy policy language.

For more information about interest-based advertising, you can also visit the Inside AdSense Blog at http://adsense.blogspot.com/2009/03/driving-monetization-with-ads-that.html.