Mission Critical is the eighth novel by Mark Greaney, published on February 19, 2019. It is also the eighth book in the Gray Man series. The novel centers on efforts by the main character Court Gentry to track down a mole inside the CIA and later stop a biological attack on an international conference in Scotland. It debuted at number five on the New York Times bestseller list.

Mark Greaney is an American novelist focusing on thriller. He is best known as Tom Clancy's collaborator on his final books during his lifetime, and for continuing the Jack Ryan character and the Tom Clancy universe following Clancy's death in 2013. He is also known for the Gray Man series of novels, which was produced by Netflix into a feature film.