Lots of people have a passion for musical theater. Jack Viertel has made his passion into a project.

For years, he has been excavating the history of the art form, digging up forgotten treasures that can be recreated for modern audiences.

Now, after 20 years at the helm of the Encores! program at City Center, Mr. Viertel is stepping down.

Next season — “Mack and Mabel,” “Love Life” and “Thoroughly Modern Millie” — will be his last as the program’s artistic director, City Center announced on Tuesday. He will continue to advise as a consulting producer, but will turn over control of the program to someone else.