Obama's chief counterterrorism adviser told PBS he believes the country is "without a doubt" safer today than it was ten years ago.
He said it has become much more difficult for terrorists to operate within the United States.
Brennan praised better interaction of government agencies and progress overseas for making the country safer.
Brennan said he doesn't believe another attack is "inevitable."
"I like to think that every day we're going to try to uncover any type of terrorist activity that's out there and take the steps to thwart any type of planning that might result in an attempted attack," said Brennan.