Denial is probably one of the best known defense mechanisms, used often to describe situations in which people seem unable to face reality or admit an obvious truth (i.e. "He's in denial."). Denial is an outright refusal to admit or recognize that something has occurred or is currently occurring. Drug addicts or alcoholics often deny that they have a problem, while victims of traumatic events may deny that the event ever occurred.
Denial functions to protect the ego from things that the individual cannot cope with. While this may save us from anxiety or pain, denial also requires a substantial investment of energy. Because of this, other defenses are also used to keep these unacceptable feelings from consciousness.