Individuals with extreme overvalued beliefs often carry out abhorrent and inexplicable acts of violence. They hold odd and bizarre beliefs that are shared by others in their culture or subculture. This creates a dilemma for the forensic psychiatrist because the definition of delusion may not be adequate. We discuss the evolution of the term “overvalued idea” first described in a forensic context by neuropsychiatrist Carl Wernicke in 1892. The overvalued idea is invoked in British scholarship as a definition for beliefs seen in anorexia nervosa, morbid jealousy, paranoid litigious states, and other disorders. This is sometimes referred to as delusional disorder by psychiatrists in the United States. The concept of an extreme overvalued belief has recently been validated and is separate from an obsession or delusion. It plays an important role in identifying fixation as a warning sign in threat assessment. We use this definition in three well-known cases (i.e., Anders Breivik, John Hinckley, Jr., and Ted Kaczynski) to emphasize how form and content of beliefs are critical to understanding the mens rea in violent criminal acts. The Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, would be enhanced by the addition of extreme overvalued belief as a definition to differentiate it from idiosyncratic, fixed, false beliefs seen in delusion.