That depends on your version of Office. Office 365 offered by MainOne works best with Office 2010 and Office 2011 for Mac. Office 365 also works with slightly limited functionality for Office 2007 and Office 2008 for Mac. If you have an earlier edition of Office, you will need to update to a newer version in order to use Office 365. If you are running a different operating system, remember that you can still use Office Online, browser-base versions of your favorite Microsoft tools like Excel, Word or PowerPoint, which are included with the Online Essentials or Business Premium Plans.