That Hideous Strength: A Modern Fairy-Tale for Grown-Ups is a 1945 novel by C. S. Lewis, the final book in Lewis's theological science fiction Space Trilogy. The events of this novel follow those of Out of the Silent Planet and Perelandra and once again feature the philologist Elwin Ransom. Yet unlike the principal events of those two novels, the story takes place on Earth rather than elsewhere in the Solar System. The story involves an ostensibly scientific institute, the N.I.C.E., which is a front for sinister supernatural forces.

Clive Staples Lewis was a British writer, literary scholar, and Anglican lay theologian. He held academic positions in English literature at both Magdalen College, Oxford (1925–1954), and Magdalene College, Cambridge (1954–1963). He is best known as the author of The Chronicles of Narnia, but he is also noted for his other works of fiction, such as The Screwtape Letters and The Space Trilogy, and for his non-fiction Christian apologetics, including Mere Christianity, Miracles, and The Problem of Pain.