The national terrorism advisory system with no colors replaces the old color coded terror threat warning system put in place following 9/11.
Starting Tuesday, new alerts will be labeled either "elevated" or "imminent'.
Homeland Security Secretary Janet Napolitano says the new system will lay out what the threat is, who is being targeted and what people should do in response.
Alerts will only be issued for a specific time period, instead of being open-ended.
