A facial expression is one or more motions or positions of the muscles in the skin. These movements convey the emotional state of the individual to observers. Facial expressions are a form of nonverbal communication. They are a primary means of conveying social information among humans, but also occur in most other mammals and some other animal species. Facial expressions and their significance in the perceiver can, to some extent, vary between cultures.

Humans can adopt a facial expression to read as a voluntary action. However, because expressions are closely tied to emotion, they are more often involuntary. It can be nearly impossible to avoid expressions for certain emotions, even when it would be strongly desirable to do so;^{[citation needed]} a person who is trying to avoid insulting an individual he or she finds highly unattractive might nevertheless show a brief expression of disgust before being able to reassume a neutral expression.^{[citation needed]} Microexpressions are one example of this phenomenon. The close link between emotion and expression can also work in the other direction; it has been observed that voluntarily assuming an expression can actually cause the associated emotion (Schnall & Laird, 2003; Soussignan, 2002 as cited in Papa & Bonanno, 2008).