Bioluminescence is the production and emission of light by a living organism as the result of a chemical reaction during which chemical energy is converted to light energy. Its name is a Hybrid word, originating from the Greek bios for "living" and the Latin lumen "light".

Ninety percent of deep-sea marine life is estimated to produce bioluminescence in one form or another. Most marine light-emission belongs in the blue and green light spectrum, the wavelengths that can transmit through the seawater most easily. However, certain loose jawed fish emit red and infrared light.

Bioluminescence is a form of luminescence, or "cold light" emission; less than 20% of the light generates thermal radiation. It should not be confused with fluorescence, phosphorescence or refraction of light.

Bioluminescence is used for camouflague, attraction, repulsion, and communication. On the surface, we see this commonly in fireflies or glow worms, but in the pitch black, these lights dance brilliantly, resembling the trembling stars of the night sky. Predators can trick prey into thinking their large bodies are nothing more than a school of fish or krill. Schools of fish, in turn, have been found changing colour to communicate directions or dangers. In defence, a number of lights on a hunted creature may appear as a sudden emergence of eyes, discouraging predators. Certain colours or patterns denote willingness to mate.

Though most look extremely primitive, the bodies of some of the creatures are decorated with vibrant colours. The white shell of a nautilus is banded and spotted with hushed, warm browns and reds. Vampyroteuthis, or “The Vampire Squid from Hell,” is a brilliant red in body colour, but has bright blue lights at the end of each tentacle to confuse predators and attract prey.

When encountering a predator or prey, the photophores of Vampyroteuthis change accordingly with its posture, in some ways producing an hypnotising effect.