Gray Day

Gray Day: My Undercover Mission to Expose America's First Cyber Spy is a 2019 non-fiction book by Eric O'Neill, published by Crown Books, about his mission to collect evidence against Robert Hanssen, an employee of the Federal Bureau of Investigation (FBI) who spied for Russia.

The book's opening scene is O'Neill's December 2000 meeting with his supervisor about the mission to ensnare Hanssen. Paul Davis wrote in the Washington Times that the main focus of the book is the operation to capture Hanssen, and that it does not "dwell on his motivation other than wanting extra money" nor does it discuss in detail any of his sexual conduct.