GOOGLE INC.

-- The Internet search leader kick-started the dealmaking on April 13, when it agreed to pay $3.1 billion in cash to acquire ad-management technology company DoubleClick Inc. Google already makes billions selling small, text-based ads linked to search terms, but DoubleClick would expand its ability to deliver multimedia display ads. A combination also would extend the size of Google's ad network. The deal is pending U.S. and European regulatory approval. Critics have complained that Google would have too much control over online advertising and personal information collected on users.

YAHOO INC.

-- Two weeks after the Google-DoubleClick announcement, the No. 2 search provider agreed to snap up the 80 percent of Right Media Inc. it did not already own. The cash-and-stock deal was worth about $650 million by the time it closed in mid-July. The acquisition gave Yahoo an exchange designed to make it easier for Web publishers to show what they have to sell to online advertisers. Right Media also specializes in the display ads that Google found so attractive about DoubleClick.

-- Yahoo announced plans Sept. 4 to buy online advertising network BlueLithium Inc. for $300 million in cash. Besides operating a large ad network, BlueLithium provides tracking technology, known as "behavioral targeting," that identifies Web surfers with particular interests so the ads they see will be more interesting to them. As an example of how this works, a person who had been looking up information about home loans on the Internet would be more likely to see ads about mortgages, even while visiting unrelated sites. Yahoo expects the deal to close by the end of the year.

MICROSOFT CORP.

-- Not wanting to get left behind, the world's largest software company followed Google and Yahoo with a May 18 announcement that it's buying online advertising firm aQuantive Inc. for $6 billion. The cash deal, which closed in mid-August, represents an 85 percent premium for aQuantive. The acquired company has ad-serving technologies and tools for tracking the success of online ad campaigns and extends Microsoft's ability to sell Web ads to aQuantive's broad network of top sites. AQuantive's Avenue A/Razorfish is a well-regarded Web design and online advertising agency.

-- Microsoft announced on July 26 plans to buy for an undisclosed amount AdECN Inc., a stock market-like exchange where networks representing Web sites buy and sell ad space. Such exchanges are primarily used to sell leftover ad space, but Microsoft has said AdECN would offer more desirable ad placements as well. The deal closed Aug. 13.

-- The company also announced in May an agreement to acquire European mobile advertising firm ScreenTonic SA. Financial terms for the deal, which closed July 2, were not disclosed.

TIME WARNER INC.'S AOL LLC

-- AOL, which already runs an ad network with its 2004 purchase of Advertising.com, announced plans July 24 to buy Tacoda Inc., which delivers targeted ads based on a visitor's browsing habits. The deal closed in early September. Financial terms were not released. AOL said Tacoda's "behavioral targeting" technology would help advertisers run more relevant pitches. Visitors who read a lot on new car models, for instance, might suddenly find automotive ads follow them to Web pages on baseball.

-- AOL announced May 16 it bought a controlling interest in Adtech AG, an online advertising company, for an undisclosed sum. Adtech, based in Frankfurt, Germany, owns the Helios IQ ad serving platform, which helps Web publishers manage, serve and report on their online advertising campaigns. The move extends AOL's ad reach internationally.

-- A day earlier, AOL said it bought Third Screen Media, a mobile advertising company, for an undisclosed sum. Third Screen Media connects advertisers, Web sites and mobile phone carriers and allows ads to be managed and delivered in a variety of wireless formats, including text messaging and mobile video.