thumb|The Titanian Threadworm of [[Stanley Weinbaum's Flight on Titan, a type of BEM, cover, Avon Fantasy Reader, 1951]]
The bug-eyed monster (BEM) is an early convention of the science fiction genre. Extraterrestrials in science fiction of the 1920s through to the 1960s were often described (or depicted on covers of pulp magazines and in films) as grotesque creatures with huge, oversized or compound eyes and a lust for women, blood, or general destruction. Their ubiquity was such that authors and readers alike began referring to them as "bug-eyed monsters", "BEMs", or "bemmies". The biology of a typical bug-eyed monster (both in general terms and in terms of their invariable attraction to human women
- When Doctor Who was created, the BBC producers stated that the show would be a "hard" science fiction show, and there would be no bug-eyed monsters – a policy explicitly stated by show creator Sydney Newman. Writer Terry Nation created the Daleks for the show's second serial, with their design resembling a bug-eyed monster much to Newman's disapproval. Nation himself would describe them as being so. The Daleks' subsequent popularity with viewers would pave the way for other BEM-type monsters to appear throughout Doctor Whos history.