Lest Darkness Fall is a 1939 alternate history science fiction novel by the American author L. Sprague de Camp. Lest Darkness Fall is similar in concept to Mark Twain's A Connecticut Yankee in King Arthur's Court, but the treatment is very different.

Alternate history is a subgenre of speculative fiction in which one or more historical events have occurred but are resolved differently than in actual history. As conjecture based upon historical fact, alternate history stories propose What if? scenarios about crucial events in human history, and present outcomes very different from the historical record. Some alternate histories are considered a subgenre of science fiction, or historical fiction.