Microsoft is now powering the search engine on Yahoo's website in the US and Canada.
Yahoo's shift to Microsoft's technology is part of a partnership aimed at undercutting Google's dominance of the search market.
The two companies completed the transition yesterday, 13 months after announcing their alliance. The shift in other countries will occur in 2011 and 2012.
By leaning on Microsoft's Bing search engine, Yahoo plans to dramatically lower expenses and focus on other products as it tries to snap out of a financial funk that has depressed its stock price.
Microsoft tried to take advantage of Yahoo's problems with an unsuccessful takeover bid in 2008.
Now, Microsoft is hoping to get better at understanding people's search requests as it picks up more traffic from Yahoo.
