Many software products claim to make your networks secure, but how do you know for sure? Common Criteria (CC) is a framework for evaluating and certifying the security of IT products and systems that is recognized by governments and IT professionals around the world as a critical measure of the quality of an information technology security product. CC certification is increasingly used as one of the key decision-making criteria by local, federal, and international government agencies and is also becoming a key differentiator for many private-sector industries such as finance and healthcare. You can read more about CC on the Common Criteria site.
For more details on how the ISA firewall achieved CC Evaluation Assurance Level 4+, check out http://www.microsoft.com/isaserver/techinfo/deployment/commoncrit.mspx