BlueLithium is an advertising network focused on serving ads based on behavioral targeting of its users. As of September 2007, BlueLithium is the fifth largest ad network in the United States and the second largest in the United Kingdom, with 145 million unique visitors per month. BlueLithium was [acquired](http://www.techcrunch.com/2007/09/04/yahoo-acquires-ad-network-bluelithium/) by [Yahoo!](/organization/yahoo) in September 2007 for approximately $300 million in cash. Other companies in the display advertising space include [DoubleClick](http://www.crunchbase.com/organization/doubleclick), [Specificmedia](http://www.crunchbase.com/organization/specificmedia), [Quigo](http://www.crunchbase.com/organization/quigo), [ValueClick](/organization/valueclick), Advertising.com, [Tribal Fusion](/organization/tribalfusion), and [aQuantive](/organization/aquantive).