2015 Reprint of 1950 Edition. Full facsimile of the original edition. Not reproduced with Optical Recognition Software. This classic account of the Psychopathic Personality was first published in 1941. We reprint herein the expanded Second Edition of 1950. Cleckley describes clinical interviews with patients in a locked institution. The text is considered to be a seminal work and the most influential clinical description of psychopathology in the twentieth century. The basic elements of psychopathology outlined by Cleckley are still relevant today. The title refers to the normal “mask” that conceals the mental disorder of the psychopathic person according to Cleckley’s conceptualization. Cleckley describes the psychopathic person as outwardly a perfect mimic of a normally functioning person, able to mask or disguise the fundamental lack of internal personality structure, an internal chaos that results in repeatedly purposeful destructive behavior, often more self-destructive than destructive to others. Despite the seemingly sincere, intelligent, even charming external presentation, internally the psychopathic person does not have the ability to experience genuine emotions. Cleckley questions whether this mask of sanity is voluntarily assumed to intentionally hide the lack of internal structure, but concludes it hides a serious, but yet imprecisely unidentified, semantic neuropsychiatric defect.