The research on the relationship between mental illness and offending is considered and the PCT perspective on mental illness is described. A PCT approach to mental illness calls on therapists to understand the perspective of the mentally disordered individual, contending that the construct systems used to make sense of the world are disordered among those suffering mental illness. Gathering information on the disordered thought processes (referred to in the chapter as “construing”) of offenders diagnosed with mental illness can be used to understand the meaning of their offending, which will in turn inform treatment strategies. Three clusters of personality disorders and their link to offending behavior are reviewed and the PCT perspective on personality disordered offenders is discussed. The PCT approach is sensitive to the fact that to the personality disordered offender, their offending behavior is actually compatible with the way they view themselves, suggesting that one path for therapeutic intervention would focus on changing how they construe themselves. Research on patterns of construing in personality disordered offenders is reviewed, which focuses on both the structure and content of their construct systems. Finally, the clinical applications of the PCT approach with mentally disordered offenders are considered, which highlight the importance of individual personality factors in assessing and treating these offenders. The PCT approach espouses a multimodal therapeutic approach in which all the offender’s challenges are identified and dealt with through the most appropriate therapeutic means. A case study is presented to illustrate the PCT approach to understanding, assessing, and treating a mentally disordered offender. Figure, references