Yahoo and Microsoft announced today that Yahoo advertisers can begin their transition to Microsoft’s adCenter.

"Last week, we completed the transition of the back-end technology for English-language Yahoo! organic search results in the U.S. and Canada," says Yahoo. "This week, advertisers can start transitioning their paid Yahoo! Search Marketing account, in anticipation of the Yahoo! ad serving transition which we expect to start mid-October. We encourage you to transition your account before the ad serving transition begins, so that you are ready to reach more than 159 million searchers in the U.S. and 15 million searchers in Canada on Yahoo! Search, Bing and our partners."

Yahoo says there are 3 stages to completing the transition, which are preparing the account for transition, actually transitioning, and continuing to manage the Yahoo account.

"The last stage in the transition process will occur when Yahoo! Search ad serving moves to adCenter, which we expect to begin in mid-October and be completed by the end of October," Yahoo says. "During this period, you should expect traffic from your Yahoo! Search Marketing account to decrease, and increase in your adCenter account. But until this process is complete, you’ll still need to actively manage your Yahoo! Search Marketing account to have your ads displayed on Yahoo! and our partner sites."

There is a transition checklist available here.