One of the nation’s best-known legal scholars and political theorists will be joining the Harvard Law School faculty in the fall, its dean, Elena Kagan, announced today.

Cass R. Sunstein, a tenured professor at the University of Chicago Law School, will head a new program at Harvard on risk regulation. The program will study how law and policy deal with 21st-century hazards such as terrorism, climate change, and occupational safety.

“Cass Sunstein is the pre-eminent legal scholar of our time — the most wide-ranging, the most prolific, the most cited, and the most influential,” Ms. Kagan said.

Mr. Sunstein said he was looking forward to the challenge.

“The nation and the world are facing many unanticipated problems, and policy makers must find ways to protect people from risks without creating unanticipated side effects,” he said. “Our goals are to improve our sense of what the law is now doing — and to see how it might do better.”

Mr. Sunstein is also a regular contributor to The Chronicle Review, writing in recent years about the polarization of extremes fostered by the Internet; F.D.R.’s 1944 State of the Union address, which Mr. Sunstein called the greatest speech of the 20th century; and the importance to democracy of being exposed to multiple viewpoints. —Katherine Mangan