Last week, Microsoft announced the largest acquisition in its history, purchasing Seattle-based interactive ad firm aQuantive for $6 billion in what it describes is the next step in the evolution of its ad network.

aQuantive is the holding company for several online ad firms including Atlas, DRIVEpm and Avenue A/Razorfish, one of the largest interactive ad agencies. The deal is the latest in a string of large acquisitions in the internet ad space (Google/DoubleClick, WPP/24/7Real Media, Yahoo/Right Media) as online advertising increasingly becomes the lynchpin of many new web-based businesses.

Microsoft, which lost out on the bidding for DoubleClick, will pay an 85% premium over aQuantive’s closing share price on Thursday, the day before the deal was announced.

aQuantive CEO Brian P. McAndrews added, “Microsoft has set a leading example in prioritizing industry partnership, transparency, measurement and quality, and we look forward to combining forces and bringing the value of our combined assets to bear for the benefit of advertisers, ad agencies and publishers.”