On Friday Microsoft announced that they bought Rapt, a San Francisco-based online ad management firm, for an undisclosed sum.

The move came three days after Google’s closed on its 3.3 billion dollar, acquisition of DoubleClick.

Both acquisitions are a sign of the heated battle between the companies for a share of the growing display advertising market.

Since Google announced its intention to buy DoubleClick last April, Microsoft has spent $6 billion to buy aQuantive, an ad network that competes with Google’s AdSense, and last month made an unsolicited offer of more than $40 billion to take over Yahoo.

Microsoft sees its newest acquisition as a way to compete with Google’s DoubleClick, which serves ads for both advertisers and publishers.Rapt, an inventory management software service for publishers, Microsoft now works with half of the top 25 online publishers, including Yahoo, CNET, and Fox Interactive Media.