Deception, whether intentional or unintentional, adds an important dimension to social interaction. Given the theoretical as well as practical benefits of detecting deceptive behavior, much effort has been devoted to developing methods of measurement that can accurately depict the act of deception. In the past, the only possible ways to detect deception relied on indirect measurement of physiological indicators that are linked to the acts of deception—for example, involuntary arousal of the autonomic nervous system during lying. Based on this connection, the polygraph was developed; it detects deception by measuring changes in skin conductivity and variations in the heart rate and respiration rate. Other methods of physiological measurement, such as biofeedback and electroencephalography, psychological instruments (pencil and paper tests), analysis of facial expressions and other body movements, and evaluation of handwriting and voice, have been developed with the hope that they could accurately detect the act of deception. Much effort has been made to examine the psychometric properties of these methods. The validity of these indices remains a matter of constant debate. The main limitation of these instruments is that they can provide only an indirect measure of the acts of deception—that is, measurement of the changes in bodily status that result from lying. Also, many liars are skillful in applying countermeasures to avoid detection once they have learned the design and aim of such instruments. However, recent advances in imaging technology have enabled direct observation of the activities of the human brain during various cognitive operations, including lying.