This research tries to understand the unconscious logics that preside over the need to lie. It analyses the type of psychological vulnerability a subject counter-invests as well as the origin of the unconscious communication he establishes with those he misleads. After having drawn up an inventory of the existing metapsychological approaches, the author examines the psychological functions of lying by taking into account its role during the development of the child. Then the examination of some adult cases that occurred in a homeless resource center will explain how, some subjects protect themselves from a harassment victim background and have others live the same experience with their lies. Finally, the author examines how cases of teenagers observed within the framework of childhood protection contribute to the analysis of the intersubjective dynamics impulsed by this acte-parlé. This work demonstrates that even though this clinical configuration is difficult to observe, if the psychologist would take it into account, it could help him understand how a patient expresses a pain that he could not tell by other means.