"Someday (I Will Understand)" is a song by American singer Britney Spears. It was written by Spears and produced by Guy Sigsworth. The song was released on August 18, 2005, by Jive Records as the sole single from Spears' first extended play, Britney & Kevin: Chaotic (2005), outside the United States. In July 2004, Spears announced her engagement to American dancer Kevin Federline, later revealing she would be taking another career break to start a family. Spears wrote the song two weeks before knowing she was pregnant with her first child, Sean Preston Federline. A pop ballad, its lyrics refer to a feeling of empowerment as a pregnant woman. A remixed version of the song was included on the 2005 remix compilation, B in the Mix: The Remixes.

"Someday (I Will Understand)" has received mixed to positive reviews from music critics. A few reviewers considered it as a lesson on the singer's history and a "tear-jerker" ballad, while others considered it a non-savable "flop single". The song reached the top ten in Denmark, Sweden and Switzerland and also charted in a number of European countries. An accompanying music video, directed by Michael Haussman, premiered on the finale of Spears' reality show Britney and Kevin: Chaotic (2005). Entirely shot in black-and-white, the music video features Spears as a pregnant woman and portrays a transformation of her character.