Contrary to a general tendency in Western thought, Michel Foucault pays attention to negative structure. In almost all of his works, the voices of individuals deemed dangerous by society are heard. The mad, the drunk, the criminal, the thief, the homosexual, the prostitute are present to show the reader how power performs the work of individualization. The aim of this article is to explore the concept of dangerousness. Dangerousness is directly related to the methods of crime and punishment. To grasp the concept, the article first discusses the practice of lettre de cachet. This practice demonstrates how absolute power permeates  society. It also explains how undesirables in society are arbitrarily transformed into the other. The genealogy of the disciplinary power that developed in the following centuries has its roots here. Disciplinary power is a form of power that calculates all behaviors of individuals and imposes individuality upon them. Dangerous individuals are identified, controlled and pacified under  this model of power. In other words, dangerous individuals are constructed through the mechanism of disciplinary power. Dangerousness, which appears as a criminal procedure, transcends the legal framework to become a central theme in a much broader framework.