Pathological lying, also known as mythomania and pseudologia fantastica, is a chronic behavior in which the person habitually or compulsively lies. These lies often serve no obvious purpose other than to paint oneself as a hero or victim, depending on the circumstance. Pathological lying has been defined as: "a persistent, pervasive, and often compulsive pattern of excessive lying behavior that leads to clinically significant impairment of functioning in social, occupational, or other areas; causes marked distress; poses a risk to the self or others; and occurs for longer than six months." Others have defined pathological lying as "falsification entirely disproportionate to any discernible end in view, may be extensive and very complicated, and may manifest over a period of years or even a lifetime."

It was first described in the medical literature in 1890 by G. Stanley Hall and in 1891 by Anton Delbrück. There is still much controversy in the fields of psychology and psychiatry about whether or not pathological lying is a unique disorder or merely a symptom of other disorders. A widely agreed-upon description of or diagnostic criteria for pathological lying behaviour does not exist, resulting in controversy regarding what it truly means to be a pathological liar. Theories to explain the root causes include stress, an attempt to shift a locus of control to an internal one, and issues relating to low self-esteem.