"What'll I Do" is the name of a song written by Irving Berlin in 1923. It was introduced by singers Grace Moore and John Steel late in the run of Berlin's third Music Box Revue and also was included in the following year's (1924) edition. In the lyrics, the singer questions how he or she will get by now that a recent romance has ended.
The song was used as a generalized theme in Nelson Riddle's Academy award-winning period score for the 1974 film The Great Gatsby starring Robert Redford and Mia Farrow, sung by character actor William Atherton. (source)
It has been performed and/or recorded by numerous artists and musicians, but it would be of no surprise to any frequent visitor to EBiN that one of my favorite versions was done by Frank Sinatra. Here's the version he recorded toward the end of his Columbia Records era in 1947:
But I think my favorite version of "What'll I Do" was the very first version I ever heard back around 1984, from Linda Ronstadt, on her first of three collaborations with none other than Nelson Riddle. I've read somewhere (or seen an interview in which she says) Ronstadt wasn't sure could handle (vocally) the American Songbook/Sinatra/Swing Era type of song. Hard to believe that(!) on something like this: