In my master’s thesis I dealt with the psychological traits of offenders and the ways in which they affect the outcome of a criminal proceeding. In the introductory chapter I made an overview of the studies and theories in which the experts identify the psychological traits of criminals. Despite the established concept of punishing the offender for his act, there are law theorists and philosophers that focus on the offender's character. The first person who gave importance to the meaning of one's psychological traits was Aristotle. In the 18th century, the philosopher David Hume developed a theory that sets the character as the foundation for criminal responsibility. Hume’s ideas were further developed by the character theorists in the second half of the 20th century. In my master’s thesis the introduction into these ideas is followed by the arguments, which dismiss their potential use in practice. In the last part of the thesis, I focused on the psychological traits that can be found in Slovenian criminal legislation and case law. I divided them into five groups: psychological traits as an element of guilt, the motive for the offence, the circumstances of the offence, offender's behaviour after the act and offender's other traits.