My main object has been to demonstrate how im portant is the thorough examination of the individual offender, especially in regard to his mentality. It is only by a great extension of this line of investigation that we can hope to solve the problems which criminality presents. A considerable part of the book is devoted to that new development of psychology which is known as psycho-analysis, and to the possible applications thereof to the investigation and treatment of offenders. I have included a brief description of the theory and technique of psycho-analysis, so that the reader may not have to look elsewhere for an explanation of technical terms.