Two-headed people and animals, though rare, have long been known to exist and documented.

Polycephaly is the condition of having more than one head. The term is derived from the Egyptian stems "poly" (Greek: "πολύ") meaning "many" and "kephalē" (Greek: "κεφάλη") meaning "head". A polycephalic organism may be thought of as one being with a supernumerary body part, or as two or more beings with a shared body.

Two-headed animals (called bicephalic or dicephalic) and three-headed (tricephalic) animals are the only type of multi-headed creatures seen in the real world, and form by the same process as conjoined twins from monozygotic twin embryos.

In humans, there are two forms of twinning that can lead to two heads being supported by a single torso. In dicephalus parapagus dipus, the two heads are side by side. In craniopagus parasiticus, the two heads are joined directly to each other, but only one head has a functional torso. Survival to adulthood is rare, but does occur in some forms of dicephalus parapagus dipus.

There are many occurrences of multi-headed animals in mythology. In heraldry and vexillology, the double-headed eagle is a common symbol, though no such animal is known to have ever existed.

Supernumerary body parts are most commonly a congenital disorder involving the growth of an additional part of the body and a deviation from the body plan. Body parts may be easily visible or hidden away, such as internal organs.

Many additional body parts form by the same process as conjoined twins: the zygote begins to split but fails to completely separate. This condition may also be a symptom of repeated occurrences of continuous inbreeding in a genetic line.

Vestigial structures are anatomical structures of organisms in a species which are considered to have lost much or all of their original function through evolution. These body parts can be classed as additional to the required functioning of the body. In human anatomy the vermiform appendix is sometimes classed as a vestigial remnant.

Prosthesis is an artificial extension that replaces a body part, and cybernetics is the study of computer technology in relation to organisms which can include replacement or additional body parts.

Body integrity identity disorder (BIID) is a psychiatric disorder in which a person thinks that they have one or more additional limbs than they should, despite having two arms and two legs. People with this condition often wish to amputate what they see as additional body parts.

A phantom limb is the sensation that a missing limb is still attached to the body.

A supernumerary phantom limb is the sensation of having an extra limb or body part despite no such limb actually existing. It is an uncommon syndrome, usually due to some kind of brain injuries in the somatosensory cortex or in some parts of the right hemisphere of the brain, usually due to a stroke in the brain.

A chimera is an animal or plant that has two or more different populations of genetically distinct cells that originated in different zygotes that have merged; anatomical structures are typically mixed depending on which cells are prevalent in different body parts, for example plants can have two different types of flowers.

A mosaic is a genetic anomaly similar in nature and effects to a chimera: genetically different populations of cells within one organism, originated from some propagated mutation of a single cell rather than from outside sources.