You Never Know is a musical with a book by Rowland Leigh, adapted from the original European play By Candlelight, by Siegfried Geyer and Karl Farkas, with music by Cole Porter and Robert Katscher, lyrics by Cole Porter, additional lyrics by Leigh and Edwin Gilbert, directed by Leigh, and songs by others.

By Candlelight is a 1933 American pre-Code comedy film directed by James Whale. The film is based on the Austrian play Candle Light by Siegfried Geyer and Karl Farkas, which was adapted to the English-speaking stage by P. G. Wodehouse. The film stars Elissa Landi, Paul Lukas, Nils Asther, and Dorothy Revier. A musical version adapted by Rowland Leigh, Cole Porter, Robert Katscher and Edwin Gilbert premiered in 1938 under the title You Never Know, but was a critical and box office flop that closed after only 78 performances.