FACE, or the Freedom of Access to Clinic Entrances act, was signed into law by President Bill Clinton in 1994. It prohibits protesters at abortion clinics from blocking access to clinics or threatening those who work at the clinic or seek to use its services. The law was written in response to a rash of violent incidents at clinics, including the attempted murder of Kansas abortion doctor George Tiller (though he survived the 1993 attack, Tiller was murdered in 2009 by another anti-abortion activist). The Clinton administration justice department enforced the act fairly vigorously, but, claims PJP Fellow Carrie Johnson on NPR, the George W. Bush administration did not. Now, the Obama justice department has reinvigorated prosecution.
Since the act prohibits a form of peaceful protest, there are First Amendment issues to address. But it has been upheld in federal court on a number occasions, most recently in Ashcroft v. Norton, where the court asserted that the act actually limits conduct, not content, an important distinction for First Amendment doctrine. You can read Carrie’s piece here and the court’s opinion in Ashcroft here.