Deception is a pervasive feature of communication that can take a variety of forms. Nearly two out of every five people told a lie in the last 24 hours, and lies are especially common over email. Deception is a function performed in part through nonverbal means and is motivated by a variety of factors, sometimes to harm others but sometimes to protect others from harm or to protect relationships. Detection of deception is an additional function that we use nonverbal cues to pursue. Although people are exposed to deception frequently, and there are many nonverbal signals that could reliably distinguish truth from deception, both untrained people and professionals are poor at detecting it. Part of the errors in judgment are due to operating on stereotypes and wrong indicators, but humans also draw upon many reliable indicators. Several theories have been advanced to account for how deceptive messages are formed (the function of deception) and detected (the function of deception detection). These include the leakage hypothesis, the four-factor model, the self-presentation model, interpersonal deception theory, cognitive load theory, and truth-default theory.