Thermogenesis is the process of heat production in the metabolism of organisms. It occurs in all warm-blooded animals, and also in a few species of thermogenic plants such as the Eastern skunk cabbage, the Voodoo lily (Sauromatum venosum), and the giant water lilies of the genus Victoria. The lodgepole pine dwarf mistletoe, Arceuthobium americanum, disperses its seeds explosively through thermogenesis. Thermoregulation is an important component of a homeothermic animal's resting metabolic rate (RMR) and serves to defend body temperature within narrow limits at low or high ambient temperature. The energy used to sustain thermogenesis is obtained in cellular respiration when nutrients such as glucose or fatty acids are oxidized to generate molecules of ATP.

Depending on whether or not they are initiated through locomotion and intentional movement of the muscles, thermogenic processes can be classified as one of the following:

One method animals use to raise temperature is through shivering. When an animal shivers, almost all the energy being expended shows up as heat. While shivering does not produce useful motion, it is still valuable for raising an animal's body temperature. For example, shivering is the process by which the body temperature of hibernating mammals (such as some bats and ground squirrels) is raised as these animals emerge from hibernation.

Non-shivering thermogenesis occurs in brown adipose tissue (brown fat) that is present in almost all eutherians (swine being the only exception currently known). Brown adipose tissue has a unique uncoupling protein (thermogenin, also known as uncoupling protein 1) that allows for the synthesis of ATP to be uncoupled from the production of protons (H⁺
), thus enabling mitochondria to burn fatty acids and oxygen to generate heat. The atomic structure of human uncoupling protein 1 UCP1 has been solved by cryogenic-electron microscopy. The structure has the typical fold of a member of the SLC25 family. UCP1 is locked in a cytoplasmic-open state by guanosine triphosphate in a pH-dependent manner, preventing proton leak.

High levels of free fatty acids within cells play a pivotal role in regulating mitochondrial uncoupling by stimulating proton leak. The stimulation of beta oxidation by increased levels of hormones such as thyroid hormone or noreinephrine helps to activate non-shivering thermogenesis during cold exposure. In this process, free fatty acids (derived from triacylglycerols) remove purine (ADP, GDP and others) inhibition of thermogenin, which causes an influx of H⁺
into the matrix of the mitochondrion and bypasses the ATP synthase channel. This uncouples oxidative phosphorylation, and the energy from the proton motive force is dissipated as heat rather than producing ATP from ADP, which would store chemical energy for the body's use. Thermogenesis can also be produced by leakage of the sodium-potassium pump and the Ca²⁺
pump. Thermogenesis is contributed to by futile cycles, such as the simultaneous occurrence of lipogenesis and lipolysis or glycolysis and gluconeogenesis. In a broader context, futile cycles can be influenced by activity/rest cycles such as the Summermatter cycle.

Acetylcholine stimulates muscle to raise metabolic rate.

The low demands of thermogenesis mean that free fatty acids draw, for the most part, on lipolysis as the method of energy production.

A comprehensive list of human and mouse genes regulating cold-induced thermogenesis (CIT) in living animals (in vivo) or tissue samples (ex vivo) has been assembled and is available in CITGeneDB.