The BCS is wrapping up its 15th season of action and Athlon Sports is continuing its series of BCS rankings. We ranked the best performances of each BCS bowl game and we ranked the best teams of each BCS conference. Now, we break down the top defensive units of the BCS era (1998-present). Statistics, awards, championships and NFL talent were all considered and evaluated in order to label the Top 12 defenses of the BCS era. Teams from 2012 were not eligible, otherwise both Notre Dame or Alabama could land on this list. But until the 2012 National Championship is decided, neither belongs on this list (yet).