Google is to start targeting ads to individual users based on a study of their surfing habits.
The company already tracks users' online habits and will now serve ads that are considered most relevant to their interests.
The technology used to track users' behaviour comes from the DoubleClick, which Google bought last year for $3,2-billion.
The annoucement of the new ads, which wll begin within the next few weeks, has once again raised questions about online privacy.
Yahoo and Microsoft already serve ads based on the browser activity, but critics worry that, by examining behaviour, Google could be more invasive.