The Lost Symbol is a 2009 novel written by American writer Dan Brown. It is a thriller set in Washington, D.C., after the events of The Da Vinci Code, and relies on Freemasonry for both its recurring theme and its major characters.

Robert Langdon is a fictional character created by author Dan Brown for his Robert Langdon book series: Angels & Demons (2000), The Da Vinci Code (2003), The Lost Symbol (2009), Inferno (2013) and Origin (2017). He is a Harvard University professor of Religious Iconology and "symbology".