I Do! I Do! is a musical with a book and lyrics by Tom Jones and music by Harvey Schmidt which is based on the Jan de Hartog play The Fourposter. The two-character story spans 50 years, from 1895 to 1945, as it focuses on the ups and downs experienced by Agnes and Michael throughout their marriage. The set consists solely of their bedroom, dominated by the large fourposter bed in the center of the room.

Jan de Hartog was a Dutch playwright, novelist and occasional social critic who moved to the United States in the early 1960s and became a Quaker.