Decomposition is the process in which the organs and complex molecules of animal and human bodies break down into simple organic matter over time. In vertebrates, five stages of decomposition are typically recognized: fresh, bloat, active decay, advanced decay, and dry/skeletonized. Knowing the different stages of decomposition can help investigators in determining the post-mortem interval (PMI). The rate of decomposition of human remains can vary due to environmental factors and other factors. Environmental factors include temperature, burning, humidity, and the availability of oxygen. Other factors include body size, clothing, and the cause of death.

The five stages of decomposition—fresh (autolysis), bloat, active decay, advanced decay, and dry/skeletonized—have specific characteristics that are used to identify which stage the remains are in. These stages are illustrated by reference to an experimental study of the decay of a pig corpse.

At this stage the remains are usually intact and free of insects. The corpse progresses through algor mortis (a reduction in body temperature until ambient temperature is reached), rigor mortis (the temporary stiffening of the limbs due to chemical changes in the muscles), and livor mortis (pooling of the blood on the side of the body that is closest to the ground).

At this stage, the microorganisms residing in the digestive system begin to digest the tissues of the body, excreting gases that cause the torso and limbs to bloat, and producing foul-smelling chemicals including putrescine and cadaverine. Cells in tissues break down and release hydrolytic enzymes, and the top layer of skin may become loosened, leading to skin slippage. Decomposition of the gastrointestinal tract results in a dark, foul-smelling liquid called "purge fluid" that is forced out of the nose and mouth due to gas pressure in the intestine. The bloat stage is characterized by a shift in the bacterial population from aerobic to anaerobic bacterial species.

At this stage, the tissues begin to liquify and the skin will start to blacken. Blowflies target decomposing corpses early on, using specialized smell receptors, and lay their eggs in orifices and open wounds. The size and development stage of maggots can be used to give a measure of the minimum time since death. Insect activity occurs in a series of waves, and identifying the insects present can give additional information on the postmortem interval. Adipocere, or corpse wax, may be formed, inhibiting further decomposition.

During advanced decay, most of the remains have discolored and often blackened. Putrefaction, in which tissues and cells break down and liquidize as the body decays, will be almost complete. A decomposing human body in the earth will eventually release approximately 32 g (1.1 oz) of nitrogen, 10 g (0.35 oz) of phosphorus, 4 g (0.14 oz) of potassium, and 1 g (0.035 oz) of magnesium for every kilogram of dry body mass, making changes in the chemistry of the soil around it that may persist for years.

Once bloating has ceased, the soft tissue of remains typically collapses in on itself. At the end of active decay, the remains are often dried out and begin to skeletonize.

The climate and temperature in which a corpse decomposes can have great effect on the rate of decomposition; higher temperatures accelerate the physiological reactions in the body after death and speed up the rate of decomposition, and cooler temperatures may slow the rate of decomposition.