To get a gremlin, take a muppet, dip it in acid, and give it a gun or a knife. Okay, it's a little more complicated than that. It took a team of talented special-effects artists to bring the gremlins to life in a pre-CGI age.

The crew initially tried to put a monkey in a gremlin suit, but that proved to be too much like having an actual gremlin on set. (Source)

Tired of monkeying around (sorry, we had to), Dante and Spielberg turned to Chris Walas, who worked with Dante on Piranhas. Walas designed painstakingly detailed puppets of the devious gremlins.

The movie theater and bar scenes are packed with puppets smoking, drinking, and playing cards. Filmmakers built the sets over old swimming pools and put the puppeteers in the empty concrete basin to work their magic. (Source)

It's a good thing the pool was empty because the gremlins would have multiplied. Oh, and the puppeteers would have drowned.

All these scenes were brought together by editor Tina Hirsch. Hirsch worked with Joe Dante on Gremlins and Twilight Zone: The Movie, and she worked on volcano disaster Dante's Peak…which has nothing to do with Joe Dante. Why'd the volcano really explode? We're betting on gremlins.