Aggregate Roots

The biggest rule to keep in mind is that aggregate roots are the only kind of entity to which your software may hold a reference. 

 http://msdn.microsoft.com/en-us/magazine/dd419654.aspx

Instead of worrying too much about aggregate boundaries, we greatly reduced the relative boundaries of roots, until it was a use-case per use-case basis that determined where we drew our boundaries.

http://lostechies.com/jimmybogard/2009/09/11/wither-the-repository/ - failure of ARs