Office 365 Enterprise E1 works best with the new Office, Office 2013, and Office 2011 for Mac. Previous versions of Office, such as Office 2010 and Office 2007, may work with Office 365 with reduced functionality. Note that you do not need to have a desktop version of Office to be able to use Enterprise E1. However, you will be able to work offline better if you connect a desktop version of Office to the Office 365 services and keep them in sync.