Blubber is a thick layer of fatty tissue which is located between the muscles and the skin of marine mammals. It serves as a source of energy for these animals during lean periods, and it also insulates the animals from extreme cold. Arctic animals in particular tend to have dense deposits of blubber, since they live in an extremely cold environment. Humans have also found an assortment of uses for blubber, ranging from a vital part of the Arctic diet to a form of fuel.

Whales, seals, and polar bears, among many others, all have blubber. The material covers the torso, not extending to appendages like flippers. It occurs in varying depths around the body, depending on the type of animal and the conditions in which it is living, and at times can comprise up to 50% of the body weight of an animal. When marine mammals have difficulty finding food, they use their blubber deposits for energy, rebuilding them when food is more abundant.

The structure of blubber is slightly different than that of other types of fatty tissue. Blubber is heavily vascularized fat, which means that it is littered with an assortment of blood vessels. The thickness of blubber allows it to act as a thermal insulator for animals while also keeping the blood of the animal warm. Unlike fur, blubber does not compress under pressure, so it will hold heat more effectively than a thick fur coat.

Like other fatty tissue, blubber makes animals more buoyant, a useful trait for marine mammals. As these animals spend much of their lives in the water, blubber is very important to their overall health. If the blubber stocks of an animal are heavily depleted, the animal will not be able to survive, as it will lack energy, thermal insulation, and natural buoyancy. This is a major concern for animals such as polar bears, which are forcing serious habitat depletion which reduces available food sources, forcing the animals to metabolize their blubber.

Historically, Arctic humans have also relied on blubber. It has served as a source of food for many people in Northern regions of the world, and it has also been burned as fuel to heat and light residences. Blubber is removed from marine mammals in long strips which are torn off with blubber knives, and then it is rendered in large pots. The rendering process is quite dirty and smelly, leading many people to be thankful that blubber is no longer a major source of food and fuel.