A mirror neuron is a neuron that fires both when an animal acts and when the animal observes the same action performed by another. Thus, the neuron "mirrors" the behavior of the other, as though the observer were itself acting. Mirror neurons are not always physiologically distinct from other types of neurons in the brain; their main differentiating factor is their response patterns. By this definition, such neurons have been directly observed in humans and other primates, as well as in birds.

In humans, brain activity consistent with that of mirror neurons has been found in the premotor cortex, the supplementary motor area, the primary somatosensory cortex, and the inferior parietal cortex. The function of the mirror system in humans is a subject of much speculation. Birds have been shown to have imitative resonance behaviors and neurological evidence suggests the presence of some form of mirroring system. To date, no widely accepted neural or computational models have been put forward to describe how mirror neuron activity supports cognitive functions.

The subject of mirror neurons continues to generate intense debate. In 2014, Philosophical Transactions of the Royal Society B published a special issue entirely devoted to mirror neuron research. Some researchers speculate that mirror systems may simulate observed actions, and thus contribute to theory of mind skills, while others relate mirror neurons to language abilities. Neuroscientists such as Marco Iacoboni have argued that mirror neuron systems in the human brain help humans understand the actions and intentions of other people. In addition, Iacoboni has argued that mirror neurons are the neural basis of the human capacity for emotions such as empathy.

In the 1980s and 1990s, neurophysiologists Giacomo Rizzolatti, Giuseppe Di Pellegrino, Luciano Fadiga, Leonardo Fogassi, and Vittorio Gallese at the University of Parma placed electrodes in the ventral premotor cortex of the macaque monkey to study neurons specialized in the control of hand and mouth actions; for example, taking hold of an object and manipulating it. During each experiment, the researchers allowed the monkey to reach for pieces of food, and recorded from single neurons in the monkey's brain, thus measuring the neuron's response to certain movements. They found that some neurons responded when the monkey observed a person picking up a piece of food, and also when the monkey itself picked up the food. The discovery was initially submitted to Nature, but was rejected for its "lack of general interest" before being published in a less competitive journal.

A few years later, the same group published another empirical paper, discussing the role of the mirror-neuron system in action recognition, and proposing that the human Broca's area was the homologue region of the monkey ventral premotor cortex. While these papers reported the presence of mirror neurons responding to hand actions, a subsequent study by Pier Francesco Ferrari and colleagues described the presence of mirror neurons responding to mouth actions and facial gestures.

Further experiments confirmed that about 10% of neurons in the monkey inferior frontal and inferior parietal cortex have "mirror" properties and give similar responses to performed hand actions and observed actions. In 2002 Christian Keysers and colleagues reported that, in both humans and monkeys, the mirror system also responds to the sound of actions.

Reports on mirror neurons have been widely published and confirmed with mirror neurons found in both inferior frontal and inferior parietal regions of the brain. Recently, evidence from functional neuroimaging strongly suggests that humans have similar mirror neurons systems: researchers have identified brain regions which respond during both action and observation of action. Not surprisingly, these brain regions include those found in the macaque monkey. However, functional magnetic resonance imaging (fMRI) can examine the entire brain at once and suggests that a much wider network of brain areas shows mirror properties in humans than previously thought. These additional areas include the somatosensory cortex and are thought to make the observer feel what it feels like to move in the observed way.

Many implicitly assume that the mirroring function of mirror neurons is due primarily to heritable genetic factors and that the genetic predisposition to develop mirror neurons evolved because they facilitate action understanding. In contrast, a number of theoretical accounts argue that mirror neurons could simply emerge due to learned associations, including the Hebbian theory, the associative learning theory, and canalization.