That Hideous Strength (subtitled 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 (also titled Voyage to Venus) 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 in space or on other planets in the solar system. The story involves an ostensibly scientific institute, the N.I.C.E., which is a front for sinister supernatural forces.

The novel was heavily influenced by the writing of Lewis's friend and fellow Inkling Charles Williams, and is markedly dystopian in style. In the book's preface Lewis acknowledges science-fiction writer Olaf Stapledon and his work: "Mr. Stapledon is so rich in invention that he can well afford to lend, and I admire his invention (though not his philosophy) so much that I should feel no shame to borrow."[2]

In the foreword, Lewis states that the novel's point is the same as that in his non-fiction work The Abolition of Man, which argues that there are natural laws and objective values, which education should teach children to recognise.

The novel's title is taken from a poem written by David Lyndsay in 1555, Ane Dialog betuix Experience and ane Courteour, also known as The Monarche. The couplet in question, "The shadow of that hyddeous strength, sax myle and more it is of length", refers to the Tower of Babel.[3]