President Donald Trump has banned travel to the U.S. by citizens of 12 countries and restricted access for those from seven others, citing national security concerns in resurrecting and expanding a hallmark policy from his first term that will mostly affect people from Africa and the Middle East.

The ban, which revives a controversial measure from Trump's first term, took effect Monday morning. Here's what to know about who's affected, who's exempt and how foreign leaders are responding.