Everyone knows what camouflage is and how it works. And many people (some scientists included) think chameleons are the masters of color change. Wrong on both counts. In this primer, I provide an overview of recent work on the mechanisms and principles of rapid adaptive camouflage of cephalopods — octopus, cuttlefish and squids. These strange but capable marine invertebrates can camouflage themselves against almost any background, a feat well appreciated by Aristotle, and one never mastered by any land animal.